PERLFUNC(1)

💖 PERLFUNC(1) — perlfunc: Perl builtin functions 💖

Perl Programmers Reference Guide

✨ NAME ✨

okay so bestie, this is perlfunc — Perl builtin functions. that's the whole name vibe. periodt. 💅

💅 DESCRIPTION 💅

okay so listen up. the functions in this section can serve as terms in an expression. they fall into two major categories: list operators and named unary operators. these differ in their precedence relationship with a following comma. (see the precedence table in perlop.) list operators take more than one argument, while unary operators can never take more than one argument. so a comma terminates the argument of a unary operator, but merely separates the arguments of a list operator. a unary operator generally provides scalar context to its argument, while a list operator may provide either scalar or list contexts for its arguments. if it does both, scalar arguments come first and list argument follow, and there can only ever be one such list argument. for instance, splice has three scalar arguments followed by a list, whereas gethostbyname has four scalar arguments. no cap. ✨

in the syntax descriptions that follow, list operators that expect a list (and provide list context for elements of the list) are shown with LIST as an argument. such a list may consist of any combination of scalar arguments or list values; the list values will be included in the list as if each individual element were interpolated at that point in the list, forming a longer single-dimensional list value. commas should separate literal elements of the LIST. 🦋

any function in the list below may be used either with or without parentheses around its arguments. (the syntax descriptions omit the parentheses.) if you use parentheses, the simple but occasionally surprising rule is this: it looks like a function, therefore it is a function, and precedence doesn't matter. otherwise it's a list operator or unary operator, and precedence does matter. whitespace between the function and left parenthesis doesn't count, so sometimes you need to be careful:

print 1+2+4;      # Prints 7.
print(1+2) + 4;   # Prints 3.
print (1+2)+4;    # Also prints 3!
print +(1+2)+4;   # Prints 7.
print ((1+2)+4);  # Prints 7.

if you run Perl with the use warnings pragma, it can warn you about this. for example, the third line above produces:

print (...) interpreted as function at - line 1.
Useless use of integer addition in void context at - line 1.

a few functions take no arguments at all, and therefore work as neither unary nor list operators. these include such functions as time and endpwent. for example, time+86_400 always means time() + 86_400. 👑

for functions that can be used in either a scalar or list context, nonabortive failure is generally indicated in scalar context by returning the undefined value, and in list context by returning the empty list. 😭

remember the following important rule: there is no rule that relates the behavior of an expression in list context to its behavior in scalar context, or vice versa. it might do two totally different things. each operator and function decides which sort of value would be most appropriate to return in scalar context. some operators return the length of the list that would have been returned in list context. some operators return the first value in the list. some operators return the last value in the list. some operators return a count of successful operations. in general, they do what you want, unless you want consistency. 💀

a named array in scalar context is quite different from what would at first glance appear to be a list in scalar context. you can't get a list like (1,2,3) into being in scalar context, because the compiler knows the context at compile time. it would generate the scalar comma operator there, not the list concatenation version of the comma. that means it was never a list to start with. 🙈

in general, functions in Perl that serve as wrappers for system calls (syscalls) of the same name (like chown(2), fork(2), closedir(2), etc.) return true when they succeed and undef otherwise, as is usually mentioned in the descriptions below. this is different from the C interfaces, which return -1 on failure. exceptions to this rule include wait, waitpid, and syscall. system calls also set the special $! variable on failure. other functions do not, except accidentally. 🫶

extension modules can also hook into the Perl parser to define new kinds of keyword-headed expression. these may look like functions, but may also look completely different. the syntax following the keyword is defined entirely by the extension. if you are an implementor, see PL_keyword_plugin in perlapi for the mechanism. if you are using such a module, see the module's documentation for details of the syntax that it defines. 🧚

🌈 Perl Functions by Category 🌈

here are Perl's functions (including things that look like functions, like some keywords and named operators) arranged by category. some functions appear in more than one place. any warnings, including those produced by keywords, are described in perldiag and warnings. 🌸

🦋 Functions for SCALARs or strings 🦋

chomp, chop, chr, crypt, fc, hex, index, lc, lcfirst, length, oct, ord, pack, q//, qq//, reverse, rindex, sprintf, substr, tr///, uc, ucfirst, y///

fc is available only if the fc feature is enabled or if it is prefixed with CORE::. the fc feature is enabled automatically with a use v5.16 (or higher) declaration in the current scope. ✨

🎀 Regular expressions and pattern matching 🎀

m//, pos, qr//, quotemeta, s///, split, study

🔢 Numeric functions 🔢

abs, atan2, cos, exp, hex, int, log, oct, rand, sin, sqrt, srand

🍓 Functions for real @ARRAYs 🍓

each, keys, pop, push, shift, splice, unshift, values

🌈 Functions for list data 🌈

all, any, grep, join, map, qw//, reverse, sort, unpack

💜 Functions for real %HASHes 💜

delete, each, exists, keys, values

📥 Input and output functions 📤

binmode, close, closedir, dbmclose, dbmopen, die, eof, fileno, flock, format, getc, print, printf, read, readdir, readline, rewinddir, say, seek, seekdir, select, syscall, sysread, sysseek, syswrite, tell, telldir, truncate, warn, write

say is available only if the say feature is enabled or if it is prefixed with CORE::. the say feature is enabled automatically with a use v5.10 (or higher) declaration in the current scope. 💅

📦 Functions for fixed-length data or records 📦

pack, read, syscall, sysread, sysseek, syswrite, unpack, vec

📁 Functions for filehandles, files, or directories 📁

-X, chdir, chmod, chown, chroot, fcntl, glob, ioctl, link, lstat, mkdir, open, opendir, readlink, rename, rmdir, select, stat, symlink, sysopen, umask, unlink, utime

break, caller, continue, die, do, dump, eval, evalbytes, exit, __FILE__, goto, last, __LINE__, method, next, __PACKAGE__, redo, return, sub, __SUB__, wantarray

break is available only if you enable the experimental switch feature or use the CORE:: prefix. the switch feature also enables the default, given and when statements, which are documented in Switch Statements in perlsyn. the switch feature is enabled automatically with a use v5.10 (or higher) declaration in the current scope. in Perl v5.14 and earlier, continue required the switch feature, like the other keywords. 😳

evalbytes is only available with the evalbytes feature (see feature) or if prefixed with CORE::. __SUB__ is only available with the current_sub feature or if prefixed with CORE::. both the evalbytes and current_sub features are enabled automatically with a use v5.16 (or higher) declaration in the current scope. 🔥

caller, class, field, import, local, my, our, package, state, use

state is available only if the state feature is enabled or if it is prefixed with CORE::. the state feature is enabled automatically with a use v5.10 (or higher) declaration in the current scope. 🌟

🎲 Miscellaneous functions 🎲

defined, formline, lock, prototype, reset, scalar, undef

⚙️ Functions for processes and process groups ⚙️

alarm, exec, fork, getpgrp, getppid, getpriority, kill, pipe, qx//, readpipe, setpgrp, setpriority, sleep, system, times, wait, waitpid

do, import, no, package, require, use

bless, class, __CLASS__, dbmclose, dbmopen, field, method, package, ref, tie, tied, untie, use

🔌 Low-level socket functions 🔌

accept, bind, connect, getpeername, getsockname, getsockopt, listen, recv, send, setsockopt, shutdown, socket, socketpair

🧬 System V interprocess communication functions 🧬

msgctl, msgget, msgrcv, msgsnd, semctl, semget, semop, shmctl, shmget, shmread, shmwrite

👤 Fetching user and group info 👤

endgrent, endhostent, endnetent, endpwent, getgrent, getgrgid, getgrnam, getlogin, getpwent, getpwnam, getpwuid, setgrent, setpwent

🌐 Fetching network info 🌐

endprotoent, endservent, gethostbyaddr, gethostbyname, gethostent, getnetbyaddr, getnetbyname, getnetent, getprotobyname, getprotobynumber, getprotoent, getservbyname, getservbyport, getservent, sethostent, setnetent, setprotoent, setservent

gmtime, localtime, time, times

🚫 Non-function keywords 🚫

ADJUST, and, AUTOLOAD, BEGIN, catch, CHECK, cmp, CORE, __DATA__, default, defer, DESTROY, else, elseif, elsif, END, __END__, eq, finally, for, foreach, ge, given, gt, if, INIT, isa, le, lt, ne, not, or, try, UNITCHECK, unless, until, when, while, x, xor

🌍 Portability 🌍

Perl was born in Unix and can therefore access all common Unix system calls. in non-Unix environments, the functionality of some Unix system calls may not be available or details of the available functionality may differ slightly. the Perl functions affected by this are:

…and the rest of the override-able besties, continuing the roll call:

-X, binmode, chmod, chown, chroot, crypt, dbmclose, dbmopen, dump, endgrent, endhostent, endnetent, endprotoent, endpwent, endservent, exec, fcntl, flock, fork, getgrent, getgrgid, gethostbyname, gethostent, getlogin, getnetbyaddr, getnetbyname, getnetent, getppid, getpgrp, getpriority, getprotobynumber, getprotoent, getpwent, getpwnam, getpwuid, getservbyport, getservent, getsockopt, glob, ioctl, kill, link, lstat, msgctl, msgget, msgrcv, msgsnd, open, pipe, readlink, rename, select, semctl, semget, semop, setgrent, sethostent, setnetent, setpgrp, setpriority, setprotoent, setpwent, setservent, setsockopt, shmctl, shmget, shmread, shmwrite, socket, socketpair, stat, symlink, syscall, sysopen, system, times, truncate, umask, unlink, utime, wait, waitpid

If you wanna know how portable any of these are across platforms, go read perlport plus whatever platform-specific docs you've got lying around. ✨

🌈 Alphabetical Listing of Perl Functions 🌈

💅 `-X FILEHANDLE`

💅 `-X EXPR`

💅 `-X DIRHANDLE`

💅 `-X`

Okay bestie, this one's the file test operator — where X is one of the letters listed below. It's a unary operator, takes exactly one argument (a filename, a filehandle, or a dirhandle), and it tests that file to see if something is true about it. If you omit the argument, it tests $_ — except for -t, which tests STDIN instead. Unless a specific test says otherwise, you get 1 for true and '' for false. And if the file doesn't exist or can't be examined? You get back "undef" and $! (errno) gets set. 🫶

Heads up: with the exception of the -l test, all of these follow symbolic links, because they use stat() and not lstat(). So a dangling symlink can't be examined and will just report failure. 💀

Despite the silly little names, precedence is exactly the same as any other named unary operator. Here's the full menu:

-r  File is readable by effective uid/gid.
-w  File is writable by effective uid/gid.
-x  File is executable by effective uid/gid.
-o  File is owned by effective uid.

-R  File is readable by real uid/gid.
-W  File is writable by real uid/gid.
-X  File is executable by real uid/gid.
-O  File is owned by real uid.

-e  File exists.
-z  File has zero size (is empty).
-s  File has nonzero size (returns size in bytes).

-f  File is a plain file.
-d  File is a directory.
-l  File is a symbolic link (false if symlinks aren't
    supported by the file system).
-p  File is a named pipe (FIFO), or Filehandle is a pipe.
-S  File is a socket.
-b  File is a block special file.
-c  File is a character special file.
-t  Filehandle is opened to a tty.

-u  File has setuid bit set.
-g  File has setgid bit set.
-k  File has sticky bit set.

-T  File is an ASCII or UTF-8 text file (heuristic guess).
-B  File is a "binary" file (opposite of -T).

-M  Script start time minus file modification time, in days.
-A  Same for access time.
-C  Same for inode change time (Unix, may differ for other
    platforms)

Example, iconic:

while (<>) {
    chomp;
    next unless -f $_;  # ignore specials
    #...
}

Important lil' gotcha: -s/a/b/ does not do a negated substitution, bestie. But -exp($foo) still works exactly like you'd expect — because only single letters right after a minus get read as file tests. 👑

These operators are exempt from the "looks like a function rule" we talked about earlier. Meaning: an opening parenthesis after the operator does not change how much of the following code counts as the argument. If you want to separate it from the code that follows, put the opening parens before the operator (and yeah, this only applies to operators with higher precedence than unary operators):

-s($file) + 1024   # probably wrong; same as -s($file + 1024)
(-s $file) + 1024  # correct

So the permission tests -r, -R, -w, -W, -x, and -X — by default they look only at the mode of the file and your uids and gids. But there can be a whole other stack of reasons you actually can't read/write/execute the thing: network filesystem access controls, ACLs (access control lists), read-only filesystems, unrecognized executable formats. So using these six specific operators to check whether some operation is possible is usually a mistake, because it's wide open to race conditions. 😭

Also: for the superuser on local filesystems, -r, -R, -w, and -W always return 1, and -x and -X return 1 if any execute bit is set in the mode. So scripts run by the superuser might need to stat the file to figure out its actual mode, or temporarily flip their effective uid to something else. 🧚

If you're using ACLs, there's a pragma called filetest that can give you more accurate results than the bare stat mode bits. Under use filetest 'access', those filetests up there instead test whether the permission can(not) be granted using the access(2) family of system calls. And plot twist — under this pragma, -x and -X may return true even when there are no execute permission bits set (and no extra execute permission ACLs either). This strangeness comes straight from the underlying system calls' definitions, not from us. 🙈 Also, because of how use filetest 'access' is implemented, the _ special filehandle won't cache the results of the file tests while that pragma is in effect. Read the filetest pragma docs for the full tea. 🍓

Now the -T and -B tests, which are basically a vibe check. Here's how they work: the first block or so of the file gets examined to see if it's valid UTF-8 that includes non-ASCII characters. If yes → it's a -T file. If not, that same portion gets scanned for odd characters — weird control codes, characters with the high bit set, that kind of thing. If more than a third of the characters are odd, it's a -B file; otherwise it's -T. Also, any file containing a zero byte in the examined portion counts as binary. (If you're inside a use locale that includes LC_CTYPE, then "odd" means anything that isn't printable or a space in the current locale.) If -T or -B gets used on a filehandle, it examines the current IO buffer instead of the first block. Both -T and -B return true on an empty file, or on a file at EOF when testing a filehandle. And because you gotta actually read the file to do a -T test, most of the time you'll want to hit it with a -f first, like: next unless -f $file && -T $file. 💖

If any of the file tests (or either the stat or lstat operator) gets handed the special filehandle that's just a solitary underline, then the stat structure from the previous file test (or stat operator) is reused, saving you a whole system call. 🌟 (This doesn't work with -t, and remember that lstat and -l leave values in the stat structure for the symbolic link, not the real file.) (Also, if the stat buffer was filled by an lstat call, -T and -B will reset it with the results of stat _.) Example:

print "Can do.\n" if -r $x || -w _ || -x _;

stat($filename);
print "Readable\n" if -r _;
print "Writable\n" if -w _;
print "Executable\n" if -x _;
print "Setuid\n" if -u _;
print "Setgid\n" if -g _;
print "Sticky\n" if -k _;
print "Text\n" if -T _;
print "Binary\n" if -B _;

As of Perl 5.10.0, as a little treat of pure syntactic sugar, you can stack file test operators — so -f -w -x $file is equivalent to -x $file && -w _ && -f _. (But it's only fancy syntax: if you use the return value of -f $file as an argument to another filetest operator, no special magic happens.) ✨

Portability tea: -X in perlport.

And to save would-be users of your code from mysterious syntax errors (we've all been there 😭), drop something like this at the top of your script:

use v5.10;  # so filetest ops can stack

💜 `abs VALUE`

💜 `abs`

Returns the absolute value of its argument. If VALUE is omitted, it uses $_. Simple as that, slay. 💅

🦋 `accept NEWSOCKET,GENERICSOCKET`

Accepts an incoming socket connect, exactly like accept(2) does. Returns the packed address if it worked, false otherwise. See the example in "Sockets: Client/Server Communication" in perlipc. 🫶

On systems that support a close-on-exec flag on files, the flag gets set for the newly opened file descriptor, as determined by the value of $^F. See "$^F" in perlvar.

⏰ `alarm SECONDS`

⏰ `alarm`

Sets things up so a SIGALRM gets delivered to this process after the specified number of wallclock seconds has elapsed. If SECONDS isn't specified, the value stored in $_ is used. (On some machines, unfortunately, the elapsed time may be up to one second less or more than what you asked for, because of how seconds get counted — and process scheduling can delay the signal delivery even further. 💀)

Only one timer can be counting at a time, bestie. Every call disables the previous timer, and you can pass an argument of 0 to cancel the previous timer without starting a new one. The returned value is the amount of time remaining on the previous timer. 👑

Want delays with finer granularity than one second? The Time::HiRes module (from CPAN, and part of the standard distribution starting in Perl 5.8) provides ualarm. You can also use Perl's four-argument version of select while leaving the first three arguments undefined, or you might be able to use the syscall interface to reach setitimer(2) if your system supports it. See perlfaq8 for the details. 🌸

It's usually a mistake to intermix alarm and sleep calls, because sleep may be internally implemented on your system with alarm. 🙈

If you wanna use alarm to time out a system call, you need an eval/die pair. You can't rely on the alarm causing the system call to fail with $! set to EINTR, because Perl sets up signal handlers to restart system calls on some systems. Using eval/die always works — modulo the caveats given in "Signals" in perlipc. 💖

eval {
    local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
    alarm $timeout;
    my $nread = sysread $socket, $buffer, $size;
    alarm 0;
};
if ($@) {
    die unless $@ eq "alarm\n";   # propagate unexpected errors
    # timed out
}
else {
    # didn't
}

need the whole lore? go read perlipc. it's all there bestie.

Portability issues: "alarm" in perlport.

✨ all BLOCK LIST ✨

okay so all runs that BLOCK for every single element of the LIST (and it locally sets $_ to each one, cute), then it truth-checks whatever the block spits out. Returns true if every element makes the block yield true, and false if even one element makes it false.

The moment any element makes the block go false, the result is decided — it short-circuits right there and doesn't even look at the rest. girl knows when to stop.

When you use it as a condition, it's basically like using grep to count that every value satisfies the condition, except for that short-circuit behaviour.

if( all { length $_ } @strings ) {
    say "Every string is non-empty";
}

is roughly equivalent to

if( @strings == grep { length $_ } @strings ) ...

This operator is only available if the "keyword_all" feature is enabled.

heads up: it's currently considered experimental, and it will throw a compile-time warning in the category "experimental::all" unless you silence that category. iykyk 💅

✨ any BLOCK LIST ✨

Same deal, opposite energy: any runs the BLOCK for each element of the LIST (locally setting $_ to each one, obviously) and truth-checks the result of that block. Returns true if at least one element makes the block yield true, or false if nothing makes it true.

As soon as any element makes the block yield true, the result is locked in. Short-circuits immediately, no further elements considered. periodt.

As a condition it's like grep, except for that short-circuit behaviour.

if( any { length $_ } @strings ) {
    say "At least one string is non-empty";
}

is roughly equivalent to

if( grep { length $_ } @strings ) ...

This operator is only available if the "keyword_any" feature is enabled.

also experimental! It'll issue a compile-time warning in the category "experimental::any" unless that category is silenced. don't say I didn't warn you 🫶

✨ atan2 Y,X ✨

Returns the arctangent of Y/X, hanging out in the range -PI to PI. math but make it range-bound.

For the tangent operation, you may use the "Math::Trig::tan" function, or just use the familiar relation:

sub tan { sin($_[0]) / cos($_[0])  }

The return value for "atan2(0,0)" is implementation-defined; go consult your atan2(3) manpage for more info. it's giving "ask your local library"

Portability issues: "atan2" in perlport.

✨ bind SOCKET,NAME ✨

Binds a network address to a socket, exactly like bind(2) does. Returns true if it worked, false otherwise. NAME should be a packed address of the appropriate type for the socket. need receipts? check the examples in "Sockets: Client/Server Communication" in perlipc.

✨ binmode FILEHANDLE, LAYER ✨

✨ binmode FILEHANDLE ✨

okay so this arranges for FILEHANDLE to be read or written in "binary" or "text" mode on systems where the run-time libraries actually distinguish between binary and text files. If FILEHANDLE is an expression, the value gets taken as the name of the filehandle. Returns true on success — otherwise it returns undef and sets $! (errno). so check your errors, bestie.

On some systems (generally the DOS- and Windows-based ones) binmode is necessary when you're not working with a text file. For portability's sake: always use it when it's appropriate, and never use it when it isn't. Also, people can set their I/O to be by default UTF8-encoded Unicode instead of bytes.

In other words: regardless of platform, use binmode on binary data. Like images. images are the moment.

If LAYER is present it's a single string, but it may contain multiple directives. Those directives alter the behaviour of the filehandle. When LAYER is present, using binmode on a text file makes sense.

If LAYER is omitted or specified as ":raw", the filehandle gets made suitable for passing binary data. That includes turning off possible CRLF translation and marking it as bytes (as opposed to Unicode characters). note: despite what may be implied in "Programming Perl" (the Camel, 3rd edition) or anywhere else, ":raw" is not simply the inverse of ":crlf". Other layers that would affect the binary nature of the stream get disabled too. See PerlIO, and the discussion about the PERLIO environment variable in perlrun.

The ":bytes", ":crlf", ":utf8", and any other directives of the form ":...", are called I/O layers. The open pragma can be used to establish default I/O layers.

The LAYER parameter of the "binmode" function is described as "DISCIPLINE" in "Programming Perl, 3rd Edition". However, since the publishing of this book — known by many as "Camel III" — the consensus on the naming of this functionality has moved from "discipline" to "layer". All documentation of this version of Perl therefore refers to "layers" rather than to "disciplines". Now back to the regularly scheduled documentation...

To mark FILEHANDLE as UTF-8, use ":utf8" or :encoding(UTF-8). ":utf8" just marks the data as UTF-8 without further checking, while :encoding(UTF-8) actually checks the data for being valid UTF-8. More details live in PerlIO::encoding.

In general, "binmode" should be called after "open" but before any I/O happens on the filehandle. Calling "binmode" normally flushes any pending buffered output data (and perhaps pending input data) on the handle. one exception: the ":encoding" layer, which changes the default character encoding of the handle. The ":encoding" layer sometimes needs to be called in mid-stream, and it doesn't flush the stream. ":encoding" also implicitly pushes on top of itself the ":utf8" layer, because internally Perl operates on UTF8-encoded Unicode characters.

The operating system, device drivers, C libraries, and the Perl run-time system all conspire to let the programmer treat a single character ("\n") as the line terminator, irrespective of external representation. On many operating systems the native text file representation matches the internal representation, but on some platforms the external representation of "\n" is made up of more than one character.

All variants of Unix, Mac OS (old and new), and Stream_LF files on VMS use a single character to end each line in the external representation of text (even though that single character is CARRIAGE RETURN on old, pre-Darwin flavors of Mac OS, and is LINE FEED on Unix and most VMS files). In other systems like OS/2, DOS, and the various flavors of MS-Windows, your program sees a "\n" as a simple "\cJ", but what's stored in text files are the two characters "\cM\cJ". That means if you don't use "binmode" on these systems, "\cM\cJ" sequences on disk get converted to "\n" on input, and any "\n" in your program gets converted back to "\cM\cJ" on output. This is what you want for text files, but it can be disastrous for binary files. disastrous. like, actually.

Another consequence of using "binmode" (on some systems) is that special end-of-file markers will be seen as part of the data stream. For systems from the Microsoft family this means that if your binary data contain "\cZ", the I/O subsystem will regard it as the end of the file — unless you use "binmode". 💀

"binmode" matters not only for "readline" and "print" operations, but also when using "read", "seek", "sysread", "syswrite" and "tell" (see perlport for more details). See the $/ and "$\" variables in perlvar for how to manually set your input and output line-termination sequences.

Portability issues: "binmode" in perlport.

✨ bless REF,CLASSNAME ✨

✨ bless REF ✨

"bless" tells Perl to mark the item referred to by REF as an object in a package. The two-argument version of "bless" is always preferable unless there's a specific reason not to use it. two args is the moment. one arg is giving risky.

Bless the referred-to item into a specific package (recommended form):

bless $ref, $package;

The two-argument form adds the object to the package specified as the second argument. slay.

Bless the referred-to item into package "main":

bless $ref, "";

If the second argument is an empty string, "bless" adds the object to package "main".

Bless the referred-to item into the current package (not inheritable):

bless $ref;

If "bless" is used without its second argument, the object is created in the current package. The second argument should always be supplied if a derived class might inherit a method executing "bless". Because it's a potential source of bugs, one-argument "bless" is discouraged. don't do it bestie.

See perlobj for more about the blessing (and blessings) of objects. yes, blessings plural. it's a whole thing. 💅

"bless" returns its first argument — the supplied reference — as the value of the function. Since "bless" is commonly the last thing executed in constructors, this means the reference to the object gets returned as the constructor's value, and it lets the caller immediately use that returned object in method calls. iconic behaviour tbh.

CLASSNAME should always be a mixed-case name, since all-uppercase and all-lowercase names are meant to be used only for Perl builtin types and pragmas, respectively. Avoid creating all-uppercase or all-lowercase package names to prevent confusion.

Also avoid "bless"ing things into the class name 0 — this will cause code which (erroneously) checks the result of "ref" to see if a reference is "bless"ed to fail, because "0", a false value, gets returned. chaos. avoid.

See "Perl Modules" in perlmod for more details.

✨ break ✨

Break out of a "given" block. escape hatch unlocked.

"break" is available only if the "switch" feature is enabled, or if it's prefixed with "CORE::". The "switch" feature is enabled automatically with a "use v5.10" (or higher) declaration in the current scope. vibes.

`caller` 📞✨

caller EXPR
caller

okay bestie, caller is literally the "who even called me rn" function and she's giving detective energy 🔍

She returns the context of the current pure perl subroutine call. In scalar context? She hands you back the caller's package name if there is a caller (aka if we're chillin' inside a subroutine, or an eval, or a require) — and the undefined value otherwise. No caller, no name, that's the vibe.

Heads up: caller never returns XS subs — they get skipped entirely. The next pure perl sub will show up instead of the XS sub in caller's return values. no cap.

In list context, caller returns this trio:

   # 0         1          2
my ($package, $filename, $line) = caller;

Just like __FILE__ and __LINE__, that filename and line number might get altered by the mechanism described at "Plain Old Comments (Not!)" in perlsyn. so don't be shocked 🌸

Now, with EXPR, she returns some extra info that the debugger uses to print a stack trace. The value of EXPR tells you how many call frames to go back before the current one. It's giving time travel 🕰️

#  0         1          2      3            4
my ($package, $filename, $line, $subroutine, $hasargs,

#  5          6          7            8       9         10
   $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
 = caller($i);

So $subroutine is the function that the caller called — not the function containing the caller, okay, that distinction matters 💅 Note that $subroutine may be "(eval)" if the frame is not a subroutine call but an eval. In that case, extra elements $evaltext and $is_require get set:

$is_require is true if the frame was created by a require or use statement
$evaltext contains the text of the eval EXPR statement

In particular, for an eval BLOCK statement, $subroutine is "(eval)", but $evaltext is undefined. (Also note: every single use statement creates a require frame inside an eval EXPR frame. layers on layers fr fr 🧅)

$subroutine may also be "(unknown)" if this particular subroutine happened to have been deleted from the symbol table. ghosted by the symbol table 😭

$hasargs is true if a new instance of @_ was set up for the frame. $hints and $bitmask contain pragmatic hints that the caller was compiled with — $hints corresponds to $^H, and $bitmask corresponds to "${^WARNING_BITS}". Those two are subject to change between versions of Perl, and are not meant for external use. so like, hands off 🫶

$hinthash is a reference to a hash containing the value of "%^H" when the caller was compiled, or "undef" if "%^H" was empty.

Do not modify the values of this hash — they are the actual values stored in the optree. touching them is a crime, and not the cute kind 💀

Note that the only types of call frames that are visible are subroutine calls and eval. Other forms of context, such as while or foreach loops or try blocks, are not considered interesting to caller, because they don't alter the behaviour of the return expression. not iconic enough for her, sorry 🎀

Furthermore, when called from within the DB package in list context, and with an argument, caller returns more detailed info: she sets the list variable @DB::args to be the arguments with which the subroutine was invoked. fancy ✨

Be aware that the optimizer might have optimized call frames away before caller had a chance to get the information. That means caller(N) might not return information about the call frame you expect it to, for "N > 1". In particular, @DB::args might have information from the previous time caller was called. the optimizer is out here deleting the receipts 😤

Be aware that setting @DB::args is best effort, intended for debugging or generating backtraces, and should not be relied upon. In particular, as @_ contains aliases to the caller's arguments, Perl does not take a copy of @_, so @DB::args will contain modifications the subroutine makes to @_ or its contents, not the original values at call time. @DB::args, like @_, does not hold explicit references to its elements, so under certain cases its elements may have become freed and reallocated for other variables or temporary values. Finally, a side effect of the current implementation is that the effects of shift @_ can normally be undone (but not pop @_ or other splicing, and not if a reference to @_ has been taken, and subject to the caveat about reallocated elements), so @DB::args is actually a hybrid of the current state and initial state of @_. Buyer beware.

`chdir` 📁🚶‍♀️

chdir EXPR
chdir FILEHANDLE
chdir DIRHANDLE
chdir

Changes the working directory to EXPR, if possible. easy peasy ✨

If EXPR is omitted, she changes to the directory specified by $ENV{HOME}, if set; if not, she changes to the directory specified by $ENV{LOGDIR}. (Under VMS, the variable $ENV{'SYS$LOGIN'} is also checked, and used if it is set.) If neither is set, chdir does nothing and fails. she just... sits there 🫠

It returns true on success, false otherwise. See the example under "die".

On systems that support fchdir(2), you may pass a filehandle or directory handle as the argument. On systems that don't support fchdir(2), passing handles raises an exception. rude but okay 💅

`chmod` 🔐💖

chmod LIST

Changes the permissions of a list of files. The first element of the list must be the numeric mode — which should probably be an octal number, and which definitely should not be a string of octal digits: 0644 is okay, but "0644" is not. that string is a trap bestie 🚨

Returns the number of files successfully changed. See also "oct" if all you have is a string.

my $cnt = chmod 0755, "foo", "bar";
chmod 0755, @executables;
my $mode = "0644"; chmod $mode, "foo";      # !!! sets mode to
                                            # --w----r-T
my $mode = "0644"; chmod oct($mode), "foo"; # this is better
my $mode = 0644;   chmod $mode, "foo";      # this is best

look at that third one just casually destroying your permissions. not the vibe 😭

On systems that support fchmod(2), you may pass filehandles among the files. On systems that don't support fchmod(2), passing filehandles raises an exception. Filehandles must be passed as globs or glob references to be recognized; barewords are considered filenames. so don't get cute 👑

open(my $fh, "<", "foo");
my $perm = (stat $fh)[2] & 07777;
chmod($perm | 0600, $fh);

You can also import the symbolic S_I* constants from the "Fcntl" module:

use Fcntl qw( :mode );
chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
# Identical to the chmod 0755 of the example above.

Portability issues: "chmod" in perlport.

`chomp` 🍓✂️

chomp VARIABLE
chomp( LIST )
chomp

This safer version of "chop" removes any trailing string that corresponds to the current value of $/ (also known as $INPUT_RECORD_SEPARATOR in the "English" module). She returns the total number of characters removed from all her arguments. 🎀

She's often used to remove the newline from the end of an input record when you're worried the final record may be missing its newline. When in paragraph mode ("$/ = ''"), she removes all trailing newlines from the string. When in slurp mode ("$/ = undef") or fixed-length record mode ($/ is a reference to an integer or the like; see perlvar), chomp won't remove anything. nothing. zero. nada 🙈

If VARIABLE is omitted, she chomps $_. Example:

while (<>) {
    chomp;  # avoid \n on last field
    my @array = split(/:/);
    # ...
}

If VARIABLE is a hash, she chomps the hash's values, but not its keys — resetting the "each" iterator in the process. yes she does that, that's just her 😌

You can actually chomp anything that's an lvalue, including an assignment:

chomp(my $cwd = `pwd`);
chomp(my $answer = <STDIN>);

If you chomp a list, each element gets chomped, and the total number of characters removed is returned. teamwork 🫶

Note that parentheses are necessary when you're chomping anything that isn't a simple variable. This is because "chomp $cwd = pwd;" is interpreted as "(chomp $cwd) = pwd;", rather than as "chomp( $cwd = pwd )" which you might expect. Similarly, "chomp $x, $y" is interpreted as "chomp($x), $y" rather than as "chomp($x, $y)". the parser is not your friend here bestie 💀

`chop` ✂️💅

chop VARIABLE
chop( LIST )
chop

Chops off the last character of a string and returns the character chopped. She is much more efficient than "s/.$//s" because she neither scans nor copies the string. efficient queen 👑

If VARIABLE is omitted, she chops $_. If VARIABLE is a hash, she chops the hash's values, but not its keys — resetting the "each" iterator in the process. (same energy as chomp, different crime 🌸)

You can actually chop anything that's an lvalue, including an assignment.

If you chop a list, each element is chopped. But only the value of the last chop is returned. so you lose the rest, rip 😭

Note that chop returns the last character. To return all but the last character, use "substr($string, 0, -1)".

`chown` 👑🔑

chown LIST

Changes the owner (and group) of a list of files. The first two elements of the list must be the numeric uid and gid, in that order. A value of -1 in either position is interpreted by most systems to leave that value unchanged. Returns the number of files successfully changed. 💖

my $cnt = chown $uid, $gid, 'foo', 'bar';
chown $uid, $gid, @filenames;

On systems that support fchown(2), you may pass filehandles among the files. On systems that don't support fchown(2), passing filehandles raises an exception. Filehandles must be passed as globs or glob references to be recognized; barewords are considered filenames. (yes, same rule as chmod, consistency slay ✨)

Here's an example that looks up nonnumeric uids in the passwd file:

print "User: ";
chomp(my $user = <STDIN>);
print "Files: ";
chomp(my $pattern = <STDIN>);

my ($login,$pass,$uid,$gid) = getpwnam($user)
    or die "$user not in passwd file";

my @ary = glob($pattern);  # expand filenames
chown $uid, $gid, @ary;

On most systems, you are not allowed to change the ownership of the file unless you're the superuser — although you should be able to change the group to any of your secondary groups. On insecure systems, these restrictions may be relaxed, but this is not a portable assumption. don't count on it 🚨

On POSIX systems, you can detect this condition this way:

use POSIX qw(pathconf _PC_CHOWN_RESTRICTED);
my $can_chown_giveaway =
    ! pathconf($path_of_interest, _PC_CHOWN_RESTRICTED);

Portability issues: "chown" in perlport.

`chr` 😊🌈

chr NUMBER
chr

Returns the character represented by that NUMBER in the character set. For example, chr(65) is "A" in either ASCII or Unicode, and chr(0x263a) is a Unicode smiley face. ☺️ iconic behavior honestly

okay so negative values? they hand you back the Unicode replacement character, chr(0xfffd) — except under the bytes pragma, where you just get the low eight bits of the value (truncated to an integer) and that's the whole story. 💀

Omit NUMBER and it grabs $_, as per usual.

Wanna go the other way? that's ord, bestie. 🫶

lowkey crucial: characters from 128 to 255 (inclusive) are by default internally not encoded as UTF-8, for backward compatibility reasons. it's giving legacy, and we respect it. 💅

More Unicode lore lives over in perlunicode if you wanna go deeper. 🦋

🌸 `chroot FILENAME`, `chroot`

Does exactly what the system call of the same name does — it makes the named directory the new root directory for all further pathnames that begin with a "/" by your process and all its children. (it does NOT change your current working directory, that one's unaffected, okay?) For security reasons, this call is restricted to the superuser. If FILENAME is omitted, it does a "chroot" to $_.

NOTE: it is mandatory for security to chdir("/") ("chdir" to the root directory) immediately after a "chroot" — otherwise your current working directory may be outside of the new root. not a vibe, don't do it. 😭

Portability issues: "chroot" in perlport.

🎀 `class NAMESPACE`, `class NAMESPACE VERSION`, `class NAMESPACE BLOCK`, `class NAMESPACE VERSION BLOCK`

Declares the BLOCK — or the rest of the compilation unit — as being in the given namespace, which implements an object class (see perlclass). This behaves similarly to "package", except that the newly-created package behaves as a class. ✨ main character package energy, honestly.

💖 `close FILEHANDLE`, `close`

Closes the file or pipe associated with the filehandle, flushes the IO buffers, and closes the system file descriptor. Returns true if those operations succeed, and if no error was reported by any PerlIO layer, and there was no existing error on the filehandle.

If there was an existing error on the filehandle, close will return false and $! will be set to the error from the failing operation — so you can safely use its value when reporting the error. 🫶

Closes the currently selected filehandle if the argument is omitted.

You don't have to close FILEHANDLE if you are immediately going to do another "open" on it, because "open" closes it for you. (See "open".) However — and this is the iconic detail — an explicit "close" on an input file resets the line counter ($.), while the implicit close done by "open" does not. 👑

If the filehandle came from a piped open, "close" returns false if one of the other syscalls involved fails or if its program exits with non-zero status. If the only problem was that the program exited non-zero, $! will be set to 0. Closing a pipe also waits for the process executing on the pipe to exit — in case you wish to look at the output of the pipe afterwards — and implicitly puts the exit status value of that command into $? and "${^CHILD_ERROR_NATIVE}". 🔥

If there are multiple threads running, "close" on a filehandle from a piped open returns true without waiting for the child process to terminate, if the filehandle is still open in another thread.

heads up: closing the read end of a pipe before the process writing to it at the other end is done writing results in the writer receiving a SIGPIPE. If the other end can't handle that, be sure to read all the data before closing the pipe. 💀

Example:

open(OUTPUT, '|sort >foo')  # pipe to sort
    or die "Can't start sort: $!";
#...                        # print stuff to output
close OUTPUT                # wait for sort to finish
    or warn $! ? "Error closing sort pipe: $!"
               : "Exit status $? from sort";
open(INPUT, 'foo')          # get sort's results
    or die "Can't open 'foo' for input: $!";

FILEHANDLE may be an expression whose value can be used as an indirect filehandle, usually the real filehandle name or an autovivified handle.

If an error occurs when perl implicitly closes a handle, perl will produce a warning. Explicitly calling close on the handle prevents that warning. 🙈

🦋 `closedir DIRHANDLE`

Closes a directory opened by "opendir" and returns the success of that system call. simple as that, slay. ✨

🌈 `connect SOCKET,NAME`

Attempts to connect to a remote socket, just like connect(2). Returns true if it succeeded, false otherwise. NAME should be a packed address of the appropriate type for the socket. See the examples in "Sockets: Client/Server Communication" in perlipc. 🧚

🍓 `continue BLOCK`, `continue`

When followed by a BLOCK, "continue" is actually a flow control statement rather than a function. If there is a "continue" BLOCK attached to a BLOCK (typically in a "while" or "foreach"), it is always executed just before the conditional is about to be evaluated again, just like the third part of a "for" loop in C. Thus it can be used to increment a loop variable, even when the loop has been continued via the "next" statement (which is similar to the C "continue" statement). 💅

"last", "next", or "redo" may appear within a "continue" block; "last" and "redo" behave as if they had been executed within the main block. So will "next", but since it will execute a "continue" block, it may be more entertaining. 😭 iconic behaviour honestly.

while (EXPR) {
    ### redo always comes here
    do_something;
} continue {
    ### next always comes here
    do_something_else;
    # then back to the top to re-check EXPR
}
### last always comes here

Omitting the "continue" section is equivalent to using an empty one, logically enough, so "next" goes directly back to check the condition at the top of the loop.

When there is no BLOCK, "continue" is a function that falls through the current "when" or "default" block instead of iterating a dynamically enclosing "foreach" or exiting a lexically enclosing "given". In Perl 5.14 and earlier, this form of "continue" was only available when the "switch" feature was enabled. See feature and "Switch Statements" in perlsyn for more information.

⭐ `cos EXPR`, `cos`

Returns the cosine of EXPR (expressed in radians). If EXPR is omitted, takes the cosine of $_. math girlies rise 🎀

For the inverse cosine operation, you may use the "Math::Trig::acos" function, or use this relation:

sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }

💜 `crypt PLAINTEXT,SALT`

Creates a digest string exactly like the crypt(3) function in the C library (assuming that you actually have a version there that has not been extirpated as a potential munition). 💀

"crypt" is a one-way hash function. The PLAINTEXT and SALT are turned into a short string, called a digest, which is returned. The same PLAINTEXT and SALT will always return the same string, but there is no (known) way to get the original PLAINTEXT from the hash. Small changes in the PLAINTEXT or SALT will result in large changes in the digest.

There is no decrypt function. This function isn't all that useful for cryptography (for that, look for Crypt modules on your nearby CPAN mirror) and the name "crypt" is a bit of a misnomer. Instead it is primarily used to check if two pieces of text are the same without having to transmit or store the text itself. An example is checking if a correct password is given. The digest of the password is stored, not the password itself. The user types in a password that is "crypt"'d with the same salt as the stored digest. If the two digests match, the password is correct. 🔐

When verifying an existing digest string you should use the digest as the salt (like crypt($plain, $digest) eq $digest). The SALT used to create the digest is visible as part of the digest. This ensures "crypt" will hash the new string with the same salt as the digest. This allows your code to work with the standard "crypt" and with more exotic implementations. In other words, assume nothing about the returned string itself nor about how many bytes of SALT may matter. 🫶

Traditionally the result is a string of 13 bytes: two first bytes of the salt, followed by 11 bytes from the set "[./0-9A-Za-z]", and only the first eight bytes of PLAINTEXT mattered. But alternative hashing schemes (like MD5), higher level security schemes (like C2), and implementations on non-Unix platforms may produce different strings.

When choosing a new salt create a random two character string whose characters come from the set "[./0-9A-Za-z]" (like join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]). This set of characters is just a recommendation; the characters allowed in the salt depend solely on your system's crypt library, and Perl can't restrict what salts "crypt" accepts.

Here's an example that makes sure that whoever runs this program knows their password:

my $pwd = (getpwuid($<))[1];

system "stty -echo";
print "Password: ";
chomp(my $word = <STDIN>);
print "\n";
system "stty echo";

if (crypt($word, $pwd) ne $pwd) {
    die "Sorry...\n";
} else {
    print "ok\n";
}

of course, typing in your own password to whoever asks you for it is unwise. 🙈 bestie no.

The "crypt" function is unsuitable for hashing large quantities of data, not least of all because you can't get the information back. Look at the Digest module for more robust algorithms.

If using "crypt" on a Unicode string (which potentially has characters with codepoints above 255), Perl tries to make sense of the situation by trying to downgrade (a copy of) the string back to an eight-bit byte string before calling "crypt" (on that copy). If that works, good. If not, "crypt" dies with "Wide character in crypt". 😭

Portability issues: "crypt" in perlport.

🌸 `dbmclose HASH`

[This function has been largely superseded by the "untie" function.]

Breaks the binding between a DBM file and a hash.

Portability issues: "dbmclose" in perlport.

🎀 `dbmopen HASH,DBNAME,MASK`

[This function has been largely superseded by the "tie" function.]

This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a hash. HASH is literally the name of the hash, bestie. (And no, unlike a normal open, the first argument is not a filehandle — even though it's giving filehandle energy, it's not one, ok?) DBNAME is the name of the database (no .dir or .pag extension if there is one). If the database doesn't exist yet, it gets created with the protection specified by MASK (as modified by the umask). Want to stop it from creating a fresh database if one isn't already there? Slap a MASK of 0 on it and the function returns a false value when it can't find an existing database. slay

If your system only supports the older DBM functions, you get one dbmopen call per program, that's it, no more, sorry. And in older versions of Perl, if your system had neither DBM nor ndbm, calling dbmopen produced a fatal error — but now it falls back to sdbm(3). glow up 💅

If you don't have write access to the DBM file, you can only read hash variables, not set them. Wanna test whether you can write? Either use file tests, or try setting a dummy hash entry inside an eval to trap the error. iykyk

Heads up bestie: functions like keys and values may return huge lists when used on large DBM files. You may prefer each to iterate over large DBM files instead. Much more chill.

Example, since we love a demo:

# print out history file offsets
dbmopen(%HIST,'/usr/lib/news/history',0666);
while (($key,$val) = each %HIST) {
    print $key, ' = ', unpack('L',$val), "\n";
}
dbmclose(%HIST);

See also AnyDBM_File for a more general description of the pros and cons of the various dbm approaches, as well as DB_File for a particularly rich implementation. ✨

And guess what — you can control which DBM library you use by loading that library before you call dbmopen:

use DB_File;
dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
    or die "Can't open netscape history file: $!";

Portability issues: "dbmopen" in perlport. 🦋

🌸 `defined EXPR` 🌸

🌸 `defined` 🌸

Returns a Boolean value telling you whether EXPR has a value that isn't the undefined value undef. If EXPR isn't present, $_ gets checked instead. periodt.

So many operations return undef to signal failure, end of file, system error, uninitialized variable, and other exceptional conditions. This function is how you tell undef apart from everything else. (Because a simple Boolean test will not distinguish among undef, zero, the empty string, and "0" — they're all equally false, and that's the trap.) Also note: since undef is a perfectly valid scalar, its presence doesn't necessarily mean something went wrong — pop returns undef when its argument is an empty array, or when the element it's returning just happens to be undef. it's giving "not a bug, just vibes"

You may also use defined(&func) to check whether subroutine func has ever been defined. The return value is unaffected by any forward declarations of func. And a subroutine that isn't defined may still be callable — its package may have an AUTOLOAD method that makes it spring into existence the first time it's called. see perlsub. 🌈

Use of defined on aggregates (hashes and arrays) is no longer supported. It used to report whether memory for that aggregate had ever been allocated. Instead, you should just do a simple test for size:

if (@an_array) { print "has array elements\n" }
if (%a_hash)   { print "has hash members\n"   }

When used on a hash element, defined tells you whether the value is defined — not whether the key exists in the hash. Use exists for that latter purpose. 💅

Examples, as requested:

print if defined $switch{D};
print "$val\n" while defined($val = pop(@ary));
die "Can't readlink $sym: $!"
    unless defined($value = readlink $sym);
sub foo { defined &$bar ? $bar->(@_) : die "No bar"; }
$debugging = 0 unless defined $debugging;

Note: many folks tend to overuse defined and are then shook to discover that the number 0 and "" (the zero-length string) are, in fact, defined values. For example, if you say

"ab" =~ /a(.*)b/;

The pattern match succeeds and $1 is defined, even though it matched "nothing". It didn't really fail to match anything — rather, it matched something that happened to be zero characters long. This is all very above-board and honest, no cap. When a function returns an undefined value, it's an admission that it couldn't give you an honest answer. So you should use defined only when you're questioning the integrity of what you're trying to do. Other times, a simple comparison to 0 or "" is what you actually want. 🎀

See also "undef", "exists", "ref". 🫶

🦋 `delete EXPR` 🦋

Give it an expression that specifies an element or slice of a hash, and delete deletes those specified elements from that hash so that exists on that element no longer returns true. Setting a hash element to the undefined value does not remove its key — but deleting it does. see "exists". big difference, bestie.

In list context, it usually returns the value or values deleted, or the last such element in scalar context. The return list's length corresponds to that of the argument list: deleting non-existent elements returns the undefined value in their corresponding positions. Since Perl 5.28, a key/value hash slice can be passed to delete, and the return value is a list of key/value pairs (two elements for each item deleted from the hash). 🔥

delete may also be used on arrays and array slices, but its behavior is less straightforward. Although exists will return false for deleted entries, deleting array elements never changes the indices of existing values — use shift or splice for that. However, if any deleted elements fall at the end of an array, the array's size shrinks to the position of the highest element that still tests true for exists, or to 0 if none do. In other words, an array won't have trailing nonexistent elements after a delete. sneaky but true

WARNING: Calling delete on array values is strongly discouraged. The notion of deleting or checking the existence of Perl array elements is not conceptually coherent, and can lead to surprising behavior. you've been warned 💀

Deleting from %ENV modifies the environment. Deleting from a hash tied to a DBM file deletes the entry from the DBM file. Deleting from a "tied" hash or array may not necessarily return anything — it depends on the implementation of the "tied" package's DELETE method, which may do whatever it pleases. 🙈

The delete local EXPR construct localizes the deletion to the current block at run time. Until the block exits, elements locally deleted temporarily no longer exist. See "Localized deletion of elements of composite types" in perlsub.

my %hash = (foo => 11, bar => 22, baz => 33);
my $scalar = delete $hash{foo};         # $scalar is 11
$scalar = delete @hash{qw(foo bar)}; # $scalar is 22
my @array  = delete @hash{qw(foo baz)}; # @array  is (undef,33)

The following (inefficiently) deletes all the values of %HASH and @ARRAY:

foreach my $key (keys %HASH) {
    delete $HASH{$key};
}

foreach my $index (0 .. $#ARRAY) {
    delete $ARRAY[$index];
}

And so do these:

delete @HASH{keys %HASH};

delete @ARRAY[0 .. $#ARRAY];

But both are slower than assigning the empty list or undefining %HASH or @ARRAY, which is the customary way to empty out an aggregate:

%HASH = ();     # completely empty %HASH
undef %HASH;    # forget %HASH ever existed

@ARRAY = ();    # completely empty @ARRAY
undef @ARRAY;   # forget @ARRAY ever existed

The EXPR can be arbitrarily complicated, as long as its final operation is an element or slice of an aggregate:

delete $ref->[$x][$y]{$key};
delete $ref->[$x][$y]->@{$key1, $key2, @morekeys};

delete $ref->[$x][$y][$index];
delete $ref->[$x][$y]->@[$index1, $index2, @moreindices];

💅 `die LIST` 💅

die raises an exception. Inside an eval, the exception gets stuffed into $@ and the eval is terminated with the undefined value. If the exception is outside of all enclosing evals, then the uncaught exception is printed to STDERR and perl exits with an exit code indicating failure. Need to exit the process with a specific exit code? see "exit". ✨

Equivalent examples:

die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"

Most of the time, die is called with a string to use as the exception. You may either give a single non-reference operand to serve as the exception, or a list of two or more items, which will be stringified and concatenated to make the exception. 🌸

If the string exception does not end in a newline, the current script line number and input line number (if any) and a newline are appended to it. Note that the "input line number" (also known as "chunk") is subject to whatever notion of "line" happens to be currently in effect, and is also available as the special variable $.. See "$/" in perlvar and "$." in perlvar. 🦋

Hint: sometimes appending ", stopped" to your message will cause it to make better sense when the string "at foo line 123" is appended. Suppose you're running script canasta.

die "/etc/games is no good";
die "/etc/games is no good, stopped";

produce, respectively

/etc/games is no good at canasta line 123.
/etc/games is no good, stopped at canasta line 123.

If LIST was empty or made an empty string, and $@ already contains an exception value (typically from a previous eval), then that value is reused after appending "\t...propagated". Super useful for propagating exceptions:

eval { ... };
die unless $@ =~ /Expected exception/;

If LIST was empty or made an empty string, and $@ contains an object reference that has a PROPAGATE method, that method will be called with additional file and line number parameters. The return value replaces the value in $@; i.e., as if "$@ = eval { $@->PROPAGATE(__FILE__, __LINE__) };" were called. 👑

If LIST was empty or made an empty string, and $@ is also empty, then the string "Died" is used. 💀

You can also hit die with a reference as its argument, and if that gets trapped inside an eval, then $@ holds that reference. Which is so useful, because it lets you do fancy elaborate exception handling with objects that carry around arbitrary state about whatever went wrong 💅. Honestly sometimes that scheme is way preferable to sitting there matching particular string values of $@ with regexes like a gremlin.

And here's the thing: because Perl stringifies uncaught exception messages before showing them, you're gonna want to overload the stringification operations on your exception objects. Go see overload for the deets on that. That stringified message should be non-empty, and it should end in a newline, so it fits in with how string exceptions get treated. Also — and this is important — because an exception object reference literally cannot be stringified without destroying it, Perl does not even attempt to append location or other info onto a reference exception 🫶. So if you want location info alongside your complex exception object, bestie, you will have to arrange to put that location info into the object yourself. No cap.

And because $@ is a global variable, be careful that analyzing an exception caught by eval doesn't replace the reference in that global. Easiest move: make a local copy of the reference before doing any manipulations. Example time:

use Scalar::Util "blessed";

eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
if (my $ev_err = $@) {
    if (blessed($ev_err)
        && $ev_err->isa("Some::Module::Exception")) {
        # handle Some::Module::Exception
    }
    else {
        # handle all other possible exceptions
    }
}

If an uncaught exception results in the interpreter exiting, the exit code gets determined from the values of $! and $? using this pseudocode:

exit $! if $!;              # errno
exit $? >> 8 if $? >> 8;    # child exit status
exit 255;                   # last resort

Just like with exit, $? is set prior to unwinding the call stack, which means any DESTROY or END handlers can then alter this value, and therefore alter Perl's exit code. Sneaky 😭.

The intent here is to squeeze as much possible information about the likely cause as it can into the limited space of the system exit code. But — and here's the catch — since $! is the value of C's errno, which can be set by literally any system call, that means the exit code value used by die can be non-predictable, so you should not rely on it. Other than the fact that it's non-zero. That's it. That's the guarantee 🌸.

You can arrange for a callback to run just before die does its deed, by setting the $SIG{__DIE__} hook. The associated handler gets called with the exception as an argument, and it can change that exception if it sees fit, by calling die again. See "%SIG" in perlvar for details on setting %SIG entries, and "eval" for some examples. So, fun story: although this feature was supposed to run only right before your program was about to exit, that is not currently so — the $SIG{__DIE__} hook is currently called even inside evaled blocks/strings! 😱 If you want the hook to do absolutely nothing in those situations, put

die @_ if $^S;

as the first line of the handler (see "$^S" in perlvar). And because this whole thing promotes strange action at a distance, this counterintuitive behavior may be fixed in a future release. Fingers crossed 🤞.

See also "exit", "warn", and the Carp module.

✨ `do BLOCK` ✨

Okay so, not really a function, bestie. It returns the value of the last command in the sequence of commands indicated by BLOCK. When it's modified by the "while" or "until" loop modifier, it executes the BLOCK once before testing the loop condition. (On other statements, the loop modifiers test the conditional first, just fyi.)

Also, do BLOCK does not count as a loop, so the loop control statements "next", "last", or "redo" cannot be used to leave or restart the block. See perlsyn for alternative strategies 💜.

✨ `do EXPR` ✨

Takes the value of EXPR, uses it as a filename, and executes the contents of that file as a Perl script:

# load the exact specified file (./ and ../ special-cased)
do '/foo/stat.pl';
do './stat.pl';
do '../foo/stat.pl';

# search for the named file within @INC
do 'stat.pl';
do 'foo/stat.pl';

do './stat.pl' is largely like

eval `cat stat.pl`;

except it's more concise, it runs no external processes, and it keeps track of the current filename for error messages. It also differs in that code evaluated with do FILE cannot see lexicals in the enclosing scope — eval STRING does. It's the same, however, in that it does reparse the file every single time you call it, so you probably do not want to be doing this inside a loop. Like, respectfully, don't 🙈.

Using do with a relative path (except for ./ and ../), like

do 'foo/stat.pl';

will search the @INC directories, and update %INC if the file is found. See "@INC" in perlvar and "%INC" in perlvar for these variables. And in particular, note that whilst historically @INC contained '.' (the current directory) — which made those two cases equivalent — that is no longer necessarily the case, since '.' is not included in @INC by default in perl versions 5.26.0 onwards. Instead, perl will now warn:

do "stat.pl" failed, '.' is no longer in @INC;
did you mean do "./stat.pl"?

If do can read the file but cannot compile it, it returns "undef" and sets an error message in $@. If do cannot read the file at all, it returns undef and sets $! to the error. Always check $@ first, because compilation could fail in a way that also sets $!. And if the file is successfully compiled, do returns the value of the last expression evaluated. Slay.

Inclusion of library modules is better done with the "use" and "require" operators, which also do automatic error checking and raise an exception if there's a problem. Just saying 💅.

You might like to use do to read in a program configuration file. Manual error checking can be done this way:

# Read in config files: system first, then user.
# Beware of using relative pathnames here.
for $file ("/share/prog/defaults.rc",
           "$ENV{HOME}/.someprogrc")
{
    unless ($return = do $file) {
        warn "couldn't parse $file: $@" if $@;
        warn "couldn't do $file: $!"    unless defined $return;
        warn "couldn't run $file"       unless $return;
    }
}

✨ `dump LABEL` ✨

✨ `dump EXPR` ✨

✨ `dump` ✨

This function causes an immediate core dump. See also the -u command-line switch in perlrun, which does the same thing. Primarily this exists so that you can use the undump program (not supplied, so like… good luck) to turn your core dump into an executable binary, after having initialized all your variables at the beginning of the program. When that new binary is executed, it will begin by executing a goto LABEL (with all the restrictions that goto suffers). Think of it as a goto with an intervening core dump and reincarnation ✨. If LABEL is omitted, it restarts the program from the top. The dump EXPR form, available starting in Perl 5.18.0, lets a name be computed at run time, and is otherwise identical to dump LABEL.

WARNING: Any files opened at the time of the dump will not be open any more when the program is reincarnated, with possible resulting confusion by Perl. That's giving chaos energy fr.

This function is now largely obsolete, mostly because it's very hard to convert a core file into an executable. And as of Perl 5.30, it must be invoked as CORE::dump().

Unlike most named operators, this one has the same precedence as assignment. It's also exempt from the looks-like-a-function rule, so dump ("foo")."bar" will cause "bar" to be part of the argument to dump. Wild, I know 💀.

Portability issues: "dump" in perlport.

✨ `each HASH` ✨

✨ `each ARRAY` ✨

When called on a hash in list context, this returns a 2-element list consisting of the key and value for the next element of a hash. When called in scalar context, it returns only the key (not the value).

When called on an array in list context, in Perl 5.12 and later, it returns a 2-element list consisting of the index and value for the next element of the array, so that you can iterate over it — older Perls consider this a syntax error, rip 😭. When called in scalar context, it returns only the index in the array.

Hash entries come back in an apparently random order. And the actual random order is specific to a given hash — the exact same series of operations on two hashes may result in a different order for each hash. Any insertion into the hash may change the order, and so will any deletion, with the exception that the most recent key returned by each or keys may be deleted without changing the order. So long as a given hash is unmodified, you may rely on keys, values and each to repeatedly return the same order as each other. See "Algorithmic Complexity Attacks" in perlsec for details on why hash order is randomized. Aside from the guarantees provided right here, the exact details of Perl's hash algorithm and the hash traversal order are subject to change in any release of Perl. So don't get attached 🦋.

Array entries are returned lowest index first.

 my @colors = (qw(red, green, blue));
 while (my ($index, $value) = each @colors) {
     print "[$index] = $value\n";
 }

 [0] = red
 [1] = green
 [2] = blue

After each has returned all entries from the hash or array, the next call to each returns the empty list in list context and undef in scalar context; the next call following that one restarts iteration. Each hash or array has its own internal iterator, accessed by each, keys, and values. That iterator is implicitly reset when each has reached the end as just described; it can be explicitly reset by calling keys or values on the hash or array, or by referencing the hash (but not array) in list context. If you add or delete a hash's elements while iterating over it, the effect on the iterator is unspecified — like, entries may be skipped or duplicated, so literally don't do that 🙈. Exception though: it is always safe to delete the item most recently returned by each, so the following code works properly:

while (my ($key, $value) = each %hash) {
    print $key, "\n";
    delete $hash{$key};   # This is safe
}

Tied hashes may have a different ordering behaviour to perl's hash implementation. so like, do not expect your tied hash to march to the same drum as the regular one, bestie 💅

The iterator used by each is attached to the hash or array, and is shared between all iteration operations applied to the same hash or array. So every single use of each on a particular hash or array advances the same iterator location. And all uses of each are also subject to having the iterator reset by any use of keys or values on the same hash or array, or by the hash (but not the array) being referenced in list context. This makes each-based loops quite fragile: it is easy to arrive at such a loop with the iterator already part way through the object, or to accidentally clobber the iterator state during execution of the loop body. It's easy enough to explicitly reset the iterator before starting a loop, but there is no way to insulate the iterator state used by a loop from the iterator state used by anything else that might execute during the loop body. 😭

This extends to using each on the result of an anonymous hash or array constructor. A new underlying array or hash is created each time so each will always start iterating from scratch, eg:

# loops forever
while (my ($key, $value) = each %{ +{ a => 1 } }) {
    print "$key=$value\n";
}

⚠️ bestie that one is an infinite loop, it says so right there in the comment, do not ship it, i am begging

To avoid these problems resulting from the hash-embedded iterator, use a foreach loop rather than while-each. As of Perl 5.36, you can iterate over both keys and values directly with a multiple-value foreach loop.

# retrieves the keys one time for iteration
# iteration is unaffected by any operations on %hash within
foreach my $key (keys %hash) {
    my $value = $hash{$key};
    $hash{$key} = {keys => scalar keys %hash, outer => [%hash]};
    some_function_that_may_mess_with(\%hash, $key, $value);
    $hash{"new$key"} = delete $hash{$key};
}

# Perl 5.36+
foreach my ($key, $value) (%{ +{ a => 1 } }) {
    print "$key=$value\n";
}

This prints out your environment like the printenv(1) program, but in a different order:

while (my ($key,$value) = each %ENV) {
    print "$key=$value\n";
}

Starting with Perl 5.14, an experimental feature allowed each to take a scalar expression. This experiment has been deemed unsuccessful, and was removed as of Perl 5.24. rip to that experiment, you will not be missed 🪦

As of Perl 5.18 you can use a bare each in a while loop, which will set $_ on every iteration. If either an each expression or an explicit assignment of an each expression to a scalar is used as a while/for condition, then the condition actually tests for definedness of the expression's value, not for its regular truth value. (iykyk, that's the sneaky part 👀)

while (each %ENV) {
    print "$_=$ENV{$_}\n";
}

To avoid confusing would-be users of your code who are running earlier versions of Perl with mysterious syntax errors, put this sort of thing at the top of your file to signal that your code will work only on Perls of a recent vintage:

use v5.12;  # so keys/values/each work on arrays
use v5.18;  # so each assigns to $_ in a lone while test

eof FILEHANDLE

eof FILEHANDLE
eof ()
eof

Returns 1 if the next read on FILEHANDLE will return end of file or if FILEHANDLE is not open. FILEHANDLE may be an expression whose value gives the real filehandle. (Note that this function actually reads a character and then ungetcs it, so isn't useful in an interactive context.) Do not read from a terminal file (or call eof(FILEHANDLE) on it) after end-of-file is reached. File types such as terminals may lose the end-of-file condition if you do. like, it just forgets. gone. poof 💀

An eof without an argument uses the last file read. Using eof() with empty parentheses is different. It refers to the pseudo file formed from the files listed on the command line and accessed via the <> operator. Since <> isn't explicitly opened, as a normal filehandle is, an eof() before <> has been used will cause @ARGV to be examined to determine if input is available. Similarly, an eof() after <> has returned end-of-file will assume you are processing another @ARGV list, and if you haven't set @ARGV, will read input from STDIN; see "I/O Operators" in perlop.

In a while (<>)" loop, eof or eof(ARGV) can be used to detect the end of each file, whereas eof() will detect the end of the very last file only. Examples:

# reset line numbering on each input file
while (<>) {
    next if /^\s*#/;  # skip comments
    print "$.\t$_";
} continue {
    close ARGV if eof;  # Not eof()!
}

# insert dashes just before last line of last file
while (<>) {
    if (eof()) {  # check for end of last file
        print "--------------\n";
    }
    print;
    last if eof();     # needed if we're reading from a terminal
}

Practical hint: you almost never need to use eof in Perl, because the input operators typically return undef when they run out of data or encounter an error. so like, save yourself the trouble bestie 🫶

eval EXPR

eval EXPR
eval BLOCK
eval

eval in all its forms is used to execute a little Perl program, trapping any errors encountered so they don't crash the calling program. it's the little safety net 🧚

Plain eval with no argument is just eval EXPR, where the expression is understood to be contained in $_. Thus there are only two real eval forms; the one with an EXPR is often called "string eval". In a string eval, the value of the expression (which is itself determined within scalar context) is first parsed, and if there were no errors, executed as a block within the lexical context of the current Perl program. This form is typically used to delay parsing and subsequent execution of the text of EXPR until run time. Note that the value is parsed every time the eval executes.

The other form is called "block eval". It is less general than string eval, but the code within the BLOCK is parsed only once (at the same time the code surrounding the eval itself was parsed) and executed within the context of the current Perl program. This form is typically used to trap exceptions more efficiently than the first, while also providing the benefit of checking the code within BLOCK at compile time. BLOCK is parsed and compiled just once. Since errors are trapped, it often is used to check if a given feature is available.

In both forms, the value returned is the value of the last expression evaluated inside the mini-program; a return statement may also be used, just as with subroutines. The expression providing the return value is evaluated in void, scalar, or list context, depending on the context of the eval itself. See wantarray for more on how the evaluation context can be determined.

If there is a syntax error or runtime error, or a die statement is executed, eval returns undef in scalar context, or an empty list in list context, and $@ is set to the error message. (Prior to 5.16, a bug caused undef to be returned in list context for syntax errors, but not for runtime errors.) If there was no error, $@ is set to the empty string. A control flow operator like last or goto can bypass the setting of $@. Beware that using eval neither silences Perl from printing warnings to STDERR, nor does it stuff the text of warning messages into $@. To do either of those, you have to use the $SIG{__WARN__} facility, or turn off warnings inside the BLOCK or EXPR using no warnings 'all'. See warn, perlvar, and warnings.

Note that, because eval traps otherwise-fatal errors, it is useful for determining whether a particular feature (such as socket or symlink) is implemented. It is also Perl's exception-trapping mechanism, where the die operator is used to raise exceptions.

Before Perl 5.14, the assignment to $@ occurred before restoration of localized variables, which means that for your code to run on older versions, a temporary is required if you want to mask some, but not all errors:

# alter $@ on nefarious repugnancy only
{
   my $e;
   {
     local $@; # protect existing $@
     eval { test_repugnancy() };
     # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
     $@ =~ /nefarious/ and $e = $@;
   }
   die $e if defined $e
}

There are some different considerations for each form:

String eval

Since the return value of EXPR is executed as a block within the lexical context of the current Perl program, any outer lexical variables are visible to it, and any package variable settings or subroutine and format definitions remain afterwards.

Note that when BEGIN {} blocks are embedded inside of an eval block the contents of the block will be executed immediately and before the rest of the eval code is executed. You can disable this entirely by

   local ${^MAX_NESTED_EVAL_BEGIN_BLOCKS} = 0;
   eval $string;

which will cause any embedded BEGIN blocks in $string to throw an exception.

Under the unicode_eval feature — if this feature is enabled (which is the default under a use 5.16 or higher declaration), Perl assumes that EXPR is a character string. Any use utf8 or no utf8 declarations within the string thus have no effect. Source filters are forbidden as well. (unicode_strings, however, can appear within the string.)

See also the evalbytes operator, which works properly with source filters.

Outside the unicode_eval feature — in this case, the behavior is problematic and is not so easily described. Here are two bugs that cannot easily be fixed without breaking existing programs:

Perl's internal storage of EXPR affects the behavior of the executed code. For example:

my $v = eval "use utf8; '$expr'";

If $expr is "\xc4\x80" (that's U+0100 in UTF-8), then whatever lands in $v depends on whether Perl is storing $expr "upgraded" (cf. utf8) or not:
- If it's upgraded, $v will be "\xc4\x80" (i.e., the "use utf8" has no effect.)
- If it's non-upgraded, $v will be "\x{100}".

This is undesirable since being upgraded or not should not affect a string's behavior. 😤

Source filters activated within "eval" leak out into whichever file scope is currently being compiled. To give an example with the CPAN module Semi::Semicolons:

 BEGIN { eval "use Semi::Semicolons; # not filtered" }
 # filtered here!

"evalbytes" fixes that to work the way one would expect:

 use feature "evalbytes";
 BEGIN { evalbytes "use Semi::Semicolons; # filtered" }
 # not filtered

Problems can arise if the string expands a scalar containing a floating point number. That scalar can expand to letters, such as "NaN" or "Infinity"; or, within the scope of a "use locale", the decimal point character may be something other than a dot (such as a comma). None of these are likely to parse as you are likely expecting. 🙈

You should be especially careful to remember what's being looked at when:

 eval $x;        # CASE 1
 eval "$x";      # CASE 2

 eval '$x';      # CASE 3
 eval { $x };    # CASE 4

 eval "\$$x++";  # CASE 5
 $$x++;          # CASE 6

Cases 1 and 2 above behave identically: they run the code contained in the variable $x. (Although case 2 has misleading double quotes making the reader wonder what else might be happening (nothing is).) Cases 3 and 4 likewise behave in the same way: they run the code '$x', which does nothing but return the value of $x. (Case 4 is preferred for purely visual reasons, but it also has the advantage of compiling at compile-time instead of at run-time.) Case 5 is a place where normally you would like to use double quotes, except that in this particular situation, you can just use symbolic references instead, as in case 6. 💅

An "eval ''" executed within a subroutine defined in the "DB" package doesn't see the usual surrounding lexical scope, but rather the scope of the first non-DB piece of code that called it. You don't normally need to worry about this unless you are writing a Perl debugger.

The final semicolon, if any, may be omitted from the value of EXPR.

🎀 Block eval 🎀

If the code to be executed doesn't vary, you may use the eval-BLOCK form to trap run-time errors without incurring the penalty of recompiling each time. The error, if any, is still returned in $@. Examples:

 # make divide-by-zero nonfatal
 eval { $answer = $x / $y; }; warn $@ if $@;

 # same thing, but less efficient
 eval '$answer = $x / $y'; warn $@ if $@;

 # a compile-time error
 eval { $answer = }; # WRONG

 # a run-time error
 eval '$answer =';   # sets $@

If you want to trap errors when loading an XS module, some problems with the binary interface (such as Perl version skew) may be fatal even with "eval" unless $ENV{PERL_DL_NONLAZY} is set. See perlrun.

Using the "eval {}" form as an exception trap in libraries does have some issues. Due to the current arguably broken state of "__DIE__" hooks, you may wish not to trigger any "__DIE__" hooks that user code may have installed. You can use the "local $SIG{__DIE__}" construct for this purpose, as this example shows:

 # a private exception trap for divide-by-zero
 eval { local $SIG{'__DIE__'}; $answer = $x / $y; };
 warn $@ if $@;

This is especially significant, given that "__DIE__" hooks can call "die" again, which has the effect of changing their error messages:

 # __DIE__ hooks may modify error messages
 {
    local $SIG{'__DIE__'} =
           sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
    eval { die "foo lives here" };
    print $@ if $@;                # prints "bar lives here"
 }

Because this promotes action at a distance, this counterintuitive behavior may be fixed in a future release.

"eval BLOCK" does not count as a loop, so the loop control statements "next", "last", or "redo" cannot be used to leave or restart the block. 🚫

The final semicolon, if any, may be omitted from within the BLOCK.

✨ `evalbytes EXPR` ✨

✨ `evalbytes` ✨

This function is similar to a string eval, except it always parses its argument (or $_ if EXPR is omitted) as a byte string. If the string contains any code points above 255, then it cannot be a byte string, and the "evalbytes" will fail with the error stored in $@. 💀

"use utf8" and "no utf8" within the string have their usual effect.

Source filters activated within the evaluated code apply to the code itself.

"evalbytes" is available starting in Perl v5.16. To access it, you must say "CORE::evalbytes", but you can omit the "CORE::" if the "evalbytes" feature is enabled. This is enabled automatically with a "use v5.16" (or higher) declaration in the current scope. 🫶

🌸 `exec LIST` 🌸

🌸 `exec PROGRAM LIST` 🌸

The "exec" function executes a system command and never returns; use "system" instead of "exec" if you want it to return. It fails and returns false only if the command does not exist and it is executed directly instead of via your system's command shell (see below).

Since it's a common mistake to use "exec" instead of "system", Perl warns you if "exec" is called in void context and if there is a following statement that isn't "die", "warn", or "exit" (if warnings are enabled--but you always do that, right?). If you really want to follow an "exec" with some other statement, you can use one of these styles to avoid the warning:

 exec ('foo')   or print STDERR "couldn't exec foo: $!";
 { exec ('foo') }; print STDERR "couldn't exec foo: $!";

If there is more than one argument in LIST, this calls execvp(3) with the arguments in LIST. If there is only one element in LIST, the argument is checked for shell metacharacters, and if there are any, the entire argument is passed to the system's command shell for parsing (this is "/bin/sh -c" on Unix platforms, but varies on other platforms). If there are no shell metacharacters in the argument, it is split into words and passed directly to "execvp", which is more efficient. Examples:

 exec '/bin/echo', 'Your arguments are: ', @ARGV;
 exec "sort $outfile | uniq";

If you don't really want to execute the first argument, but want to lie to the program you are executing about its own name, you can specify the program you actually want to run as an "indirect object" (without a comma) in front of the LIST, as in "exec PROGRAM LIST". (This always forces interpretation of the LIST as a multivalued list, even if there is only a single scalar in the list.) Example:

 my $shell = '/bin/csh';
 exec $shell '-sh';    # pretend it's a login shell

or, more directly,

 exec {'/bin/csh'} '-sh';  # pretend it's a login shell

When the arguments get executed via the system shell, results are subject to its quirks and capabilities. See "STRING" in perlop for details.

Using an indirect object with "exec" or "system" is also more secure. This usage (which also works fine with "system") forces interpretation of the arguments as a multivalued list, even if the list had just one argument. That way you're safe from the shell expanding wildcards or splitting up words with whitespace in them. 🛡️

 my @args = ( "echo surprise" );

 exec @args;               # subject to shell escapes
                            # if @args == 1
 exec { $args[0] } @args;  # safe even with one-arg list

The first version, the one without the indirect object, ran the echo program, passing it "surprise" as an argument. The second version didn't; it tried to run a program named "echo surprise", didn't find it, and set $? to a non-zero value indicating failure. 💀

On Windows, only the "exec PROGRAM LIST" indirect object syntax will reliably avoid using the shell; "exec LIST", even with more than one element, will fall back to the shell if the first spawn fails.

Perl attempts to flush all files opened for output before the exec, but this may not be supported on some platforms (see perlport). To be safe, you may need to set $| ($AUTOFLUSH in English) or call the "autoflush" method of "IO::Handle" on any open handles to avoid lost output. 🌊

Note that "exec" will not call your "END" blocks, nor will it invoke "DESTROY" methods on your objects. 😭

Portability issues: "exec" in perlport.

💜 `exists EXPR` 💜

Given an expression that specifies an element of a hash, returns true if the specified element in the hash has ever been initialized, even if the corresponding value is undef. ✨

 print "Exists\n"    if exists $hash{$key};
 print "Defined\n"   if defined $hash{$key};
 print "True\n"      if $hash{$key};

exists may also be called on array elements, but its behavior is much less obvious and is strongly tied to the use of "delete" on arrays.

WARNING: Calling "exists" on array values is strongly discouraged. The notion of deleting or checking the existence of Perl array elements is not conceptually coherent, and can lead to surprising behavior. 🙈

 print "Exists\n"    if exists $array[$index];
 print "Defined\n"   if defined $array[$index];
 print "True\n"      if $array[$index];

A hash or array element can be true only if it's defined and defined only if it exists, but the reverse doesn't necessarily hold true. 💅

okay so if you hand it an expression that names a subroutine, it gives you back true if that sub has ever been declared — even if it's undefined 💅. Lowkey important: just mentioning a sub name for exists or defined does not count as declaring it. And heads up bestie, a sub that doesn't exist might still be callable — its package could have an "AUTOLOAD" method that makes it pop into existence the first time it's called 🦋. Go read perlsub for that tea.

    print "Exists\n"  if exists &subroutine;
    print "Defined\n" if defined &subroutine;

also the EXPR can be as chaotic as it wants, as long as the final operation is a hash or array key lookup or a subroutine name:

    if (exists $ref->{A}->{B}->{$key})  { }
    if (exists $hash{A}{B}{$key})       { }

    if (exists $ref->{A}->{B}->[$ix])   { }
    if (exists $hash{A}{B}[$ix])        { }

    if (exists &{$ref->{A}{B}{$key}})   { }

now this is the sneaky part 🙈 — the most deeply nested array or hash element won't spring into existence just because you tested for it, but any intervening ones will. So "$ref->{"A"}" and "$ref->{"A"}->{"B"}" will spring into existence because of that existence test for the $key element up there. This happens anywhere the arrow operator is used, literally even here:

    undef $ref;
    if (exists $ref->{"Some key"})    { }
    print $ref;  # prints HASH(0x80d3d5c)

⚠️ and if you use an actual subroutine call instead of a subroutine name as the argument to "exists", that's an error, no cap:

    exists &sub;    # OK
    exists &sub();  # Error

🌈 exit EXPR 🌈

🌈 exit 🌈

okay bestie it evaluates EXPR and dips out immediately with that value. Example time:

    my $ans = <STDIN>;
    exit 0 if $ans =~ /^[Xx]/;

peep "die" too while you're at it. If EXPR is omitted, it exits with 0 status ✨.
The only universally recognized values for EXPR are 0 for success and 1 for error — everything else is subject to interpretation depending on the environment your Perl program is running in 🌍. Like, exiting 69 (EX_UNAVAILABLE) from a sendmail incoming-mail filter will make the mailer return the item undelivered, but that is not true everywhere, okay? iykyk.

🚨 do NOT use "exit" to abort a subroutine if there's any chance someone might want to trap whatever error happened. Use "die" instead, because that can be trapped by an "eval" 💖.

and the plot twist: "exit" does not always exit immediately 👀. It calls any defined "END" routines first, but those "END" routines may not themselves abort the exit. Likewise, any object destructors that need to be called get called before the real exit. "END" routines and destructors can change the exit status by modifying $? 😭. If that's a problem for you, you can call POSIX::_exit($status) to avoid "END" and destructor processing. See perlmod for the deets.

portability tea: "exit" in perlport.

🌸 exp EXPR 🌸

🌸 exp 🌸

Returns e (the natural logarithm base) to the power of EXPR 🧚. If EXPR is omitted, it gives you exp($_).

🦋 fc EXPR 🦋

🦋 fc 🦋

Returns the casefolded version of EXPR 💅. This is the internal function implementing the "\F" escape in double-quoted strings.

so what even is casefolding? It's the process of mapping strings to a form where case differences are erased. Comparing two strings in their casefolded form is effectively a way of asking if two strings are equal, regardless of case ✨.

Roughly — if you ever caught yourself writing this:

    lc($this) eq lc($that)    # Wrong!
        # or
    uc($this) eq uc($that)    # Also wrong!
        # or
    $this =~ /^\Q$that\E\z/i  # Right!

now you can just write:

    fc($this) eq fc($that)

And get the correct results 👑.

Perl only implements the full form of casefolding, but you can access the simple folds using "casefold()" in Unicode::UCD and "prop_invmap()" in Unicode::UCD.
For further info on casefolding, go look at the Unicode Standard, specifically sections 3.13 "Default Case Operations", 4.2 "Case-Normative", and 5.18 "Case Mappings", available at <https://www.unicode.org/versions/latest/>, plus the Case Charts at <https://www.unicode.org/charts/case/> 🌈.
If EXPR is omitted, it uses $_.

pragmas behavior check: this function behaves the same way under various pragmas, such as within "use feature 'unicode_strings", as "lc" does — with the single exception of "fc" of LATIN CAPITAL LETTER SHARP S (U+1E9E) within the scope of "use locale" 👀. The foldcase of this character would normally be "ss", but as explained in the "lc" section, case changes that cross the 255/256 boundary are problematic under locales, and are hence prohibited. Therefore, this function under locale returns instead the string "\x{17F}\x{17F}", which is the LATIN SMALL LETTER LONG S. Since that character itself folds to "s", the string of two of them together should be equivalent to a single U+1E9E when foldcased 💅.

ngl there are two more forms: while the Unicode Standard defines two additional forms of casefolding — one for Turkic languages and one that never maps one character into multiple characters — these are not provided by the Perl core. However, the CPAN module "Unicode::Casing" may be used to provide an implementation 🫶.

⚠️ "fc" is available only if the "fc" feature is enabled or if it is prefixed with "CORE::". The "fc" feature is enabled automatically with a "use v5.16" (or higher) declaration in the current scope ✨.

🎀 fcntl FILEHANDLE,FUNCTION,SCALAR 🎀

Implements the fcntl(2) function 💖. You'll probably have to say

    use Fcntl;

first to get the correct constant definitions. Argument processing and value returned work just like "ioctl" below. For example:

    use Fcntl;
    my $flags = fcntl($filehandle, F_GETFL, 0)
        or die "Can't fcntl F_GETFL: $!";

you do NOT have to check for "defined" on the return from "fcntl" — like "ioctl", it maps a 0 return from the system call into "0 but true" in Perl 💅. This string is true in boolean context and 0 in numeric context. It's also exempt from the normal "Argument "..." isn't numeric" warnings on improper numeric conversions 🙌.

heads up: "fcntl" raises an exception if used on a machine that doesn't implement fcntl(2). See the Fcntl module or your fcntl(2) manpage to learn what functions are available on your system.

here's an example of setting a filehandle named $REMOTE to be non-blocking at the system level. You'll have to negotiate $| on your own, though 😭:

    use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);

    my $flags = fcntl($REMOTE, F_GETFL, 0)
        or die "Can't get flags for the socket: $!\n";

    fcntl($REMOTE, F_SETFL, $flags | O_NONBLOCK)
        or die "Can't set flags for the socket: $!\n";

portability tea: "fcntl" in perlport.

⭐ FILE ⭐

A special token that returns the name of the file in which it occurs 👑. It can be altered by the mechanism described at "Plain Old Comments (Not!)" in perlsyn.

💜 field VARNAME 💜

Declares a new field variable within the current class 🌸. Methods and "ADJUST" blocks of the class will have access to this variable as if it was a lexical in scope at that point ✨.

🍓 fileno FILEHANDLE 🍓

🍓 fileno DIRHANDLE 🍓

Returns the file descriptor for a filehandle or directory handle, or undefined if the filehandle is not open 💅. If there is no real file descriptor at the OS level — as can happen with filehandles connected to memory objects via "open" with a reference for the third argument — then -1 is returned.

mainly useful for constructing bitmaps for "select" and low-level POSIX tty-handling operations.
If FILEHANDLE is an expression, the value is taken as an indirect filehandle, generally its name.

You can use this to find out whether two handles refer to the same underlying descriptor 👀:

    if (fileno($this) != -1 && fileno($this) == fileno($that)) {
        print "\$this and \$that are dups\n";
    } elsif (fileno($this) != -1 && fileno($that) != -1) {
        print "\$this and \$that have different " .
            "underlying file descriptors\n";
    } else {
        print "At least one of \$this and \$that does " .
            "not have a real file descriptor\n";
    }

and the vibe check on directory handles: the behavior of "fileno" on a directory handle depends on the operating system 🦋. On a system with dirfd(3) or similar, "fileno" on a directory handle returns the underlying file descriptor associated with the handle; on systems with no such support, it returns the undefined value, and sets $! (errno).

🔥 flock FILEHANDLE,OPERATION 🔥

Calls flock(2), or an emulation of it, on FILEHANDLE 💅. Returns true for success, false on failure. Produces a fatal error if used on a machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3) 😭. "flock" is Perl's portable file-locking interface, although it locks entire files only, not records 👑.

two potentially non-obvious but traditional "flock" semantics that you need to know:

it waits indefinitely until the lock is granted, and
its locks are merely advisory.

Such discretionary locks are more flexible, but offer fewer guarantees 🫶. This means that programs that do not also use "flock" may modify files locked with "flock". See perlport, your port's specific documentation, and your system-specific local manpages for details. It's best to assume traditional behavior if you're writing portable programs. (But if you're not, you should as always feel perfectly free to write for your own system's idiosyncrasies — sometimes called "features" 😌. Slavish adherence to portability concerns shouldn't get in the way of your getting your job done.)

OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with LOCK_NB ✨.
These constants are traditionally valued 1, 2, 8 and 4, but you can use the symbolic names if you import them from the Fcntl module, either individually, or as a group using the ":flock" tag 💖.
LOCK_SH requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN releases a previously requested lock.
If LOCK_NB is bitwise-or'ed with LOCK_SH or LOCK_EX, then "flock" returns immediately rather than blocking waiting for the lock — check the return status to see if you got it 👀.

to avoid the possibility of miscoordination, Perl now flushes FILEHANDLE before locking or unlocking it 💅.

Note that the emulation built with lockf(3) doesn't do shared locks, and it requires that FILEHANDLE be open with write intent. Those are literally the semantics lockf(3) implements, bestie. Most if not all systems implement lockf(3) in terms of fcntl(2) locking though, so the differing semantics shouldn't bite too many people. 🫶

Also note that the fcntl(2) emulation of flock(3) requires FILEHANDLE to be open with read intent if you wanna use LOCK_SH, and open with write intent if you wanna use LOCK_EX. Read the room, the handle has opinions.

And one more: some versions of flock cannot lock things over the network, and for that you'd need the more system-specific fcntl. If you feel like it, you can force Perl to ignore your system's flock(2) function entirely and hand you its own fcntl(2)-based emulation instead, by passing the switch -Ud_flock to the Configure program when you configure and build a new Perl. Choose chaos. 💅

Here's a mailbox appender for BSD systems:

# import LOCK_* and SEEK_END constants
use Fcntl qw(:flock SEEK_END);

sub lock {
    my ($fh) = @_;
    flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
    # and, in case we're running on a very old UNIX
    # variant without the modern O_APPEND semantics...
    seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
}

sub unlock {
    my ($fh) = @_;
    flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
}

open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
    or die "Can't open mailbox: $!";

lock($mbox);
print $mbox $msg,"\n\n";
unlock($mbox);

On systems that support a real flock(2), locks are inherited across fork calls — slay. But the ones that have to resort to the more capricious fcntl(2) function lose their locks, which makes writing servers seriously harder. 😭

See also DB_File for other flock examples.

Portability issues: "flock" in perlport.

`fork` 🌱

Does a fork(2) system call to create a new process running the same program at the same point. It returns the child pid to the parent process, 0 to the child process, or undef if the fork is unsuccessful. File descriptors (and sometimes locks on those descriptors) are shared, while everything else is copied. On most systems supporting fork(2), great care has gone into making it extremely efficient (for example, using copy-on-write technology on data pages), which is why it's been the dominant paradigm for multitasking over the last few decades. Iconic behaviour, honestly. 👑

Perl attempts to flush all files opened for output before forking the child process, but this may not be supported on some platforms (see perlport). To be safe, you may need to set $| ($AUTOFLUSH in English) or call the "autoflush" method of "IO::Handle" on any open handles to avoid duplicate output. Duplicate output is not the vibe.

If you "fork" without ever waiting on your children, you will accumulate zombies. 💀 On some systems, you can avoid this by setting $SIG{CHLD} to "IGNORE". See also perlipc for more examples of forking and reaping moribund children.

Note that if your forked child inherits system file descriptors like STDIN and STDOUT that are actually connected by a pipe or socket, even if you exit, then the remote server (such as, say, a CGI script or a backgrounded job launched from a remote shell) won't think you're done. You should reopen those to /dev/null if it's any issue.

On some platforms such as Windows, where the fork(2) system call is not available, Perl can be built to emulate "fork" in the Perl interpreter. The emulation is designed, at the level of the Perl program, to be as compatible as possible with the "Unix" fork(2). However it has limitations that have to be considered in code intended to be portable. See perlfork for more details.

Portability issues: "fork" in perlport.

`format` 🎀

Declare a picture format for use by the "write" function. For example:

format Something =
    Test: @<<<<<<<< @||||| @>>>>>
          $str,     $%,    '$' . int($num)
.

$str = "widget";
$num = $cost/$quantity;
$~ = 'Something';
write;

See perlform for many details and examples.

`formline PICTURE,LIST` 🦋

This is an internal function used by "format"s, though you may call it too. It formats (see perlform) a list of values according to the contents of PICTURE, placing the output into the format output accumulator, $^A (or $ACCUMULATOR in English). Eventually, when a "write" is done, the contents of $^A are written to some filehandle. You could also read $^A and then set $^A back to "". Note that a format typically does one "formline" per line of form, but the "formline" function itself doesn't care how many newlines are embedded in the PICTURE. This means that the "~" and "~~" tokens treat the entire PICTURE as a single line. You may therefore need to use multiple formlines to implement a single record format, just like the "format" compiler.

Be careful if you put double quotes around the picture, because an "@" character may be taken to mean the beginning of an array name. 😳 "formline" always returns true. See perlform for other examples.

If you are trying to use this instead of "write" to capture the output, you may find it easier to open a filehandle to a scalar (open my $fh, ">", \$output) and write to that instead. Lowkey a glow-up move. ✨

`getc FILEHANDLE` / `getc` 💌

Returns the next character from the input file attached to FILEHANDLE, or the undefined value at end of file or if there was an error (in the latter case $! is set). If FILEHANDLE is omitted, reads from STDIN. This is not particularly efficient. However, it cannot be used by itself to fetch single characters without waiting for the user to hit enter. For that, try something more like:

if ($BSD_STYLE) {
    system "stty cbreak </dev/tty >/dev/tty 2>&1";
}
else {
    system "stty", '-icanon', 'eol', "\001";
}

my $key = getc(STDIN);

if ($BSD_STYLE) {
    system "stty -cbreak </dev/tty >/dev/tty 2>&1";
}
else {
    system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
}
print "\n";

Determination of whether $BSD_STYLE should be set is left as an exercise to the reader. 😌 Good luck bestie.

The "POSIX::getattr" function can do this more portably on systems purporting POSIX compliance. See also the "Term::ReadKey" module on CPAN.

`getlogin` 🧚

This implements the C library function of the same name, which on most systems returns the current login from /etc/utmp, if any. If it returns the empty string, use "getpwuid".

my $login = getlogin || getpwuid($<) || "Kilroy";

Do not consider "getlogin" for authentication: it is not as secure as "getpwuid". Periodt. 🚫

Portability issues: "getlogin" in perlport.

`getpeername SOCKET` 🌈

Returns the packed sockaddr address of the other end of the SOCKET connection.

use Socket;
my $hersockaddr    = getpeername($sock);
my ($port, $iaddr) = sockaddr_in($hersockaddr);
my $herhostname    = gethostbyaddr($iaddr, AF_INET);
my $herstraddr     = inet_ntoa($iaddr);

`getpgrp PID` 🌸

Returns the current process group for the specified PID. Use a PID of 0 to get the current process group for the current process. Will raise an exception if used on a machine that doesn't implement getpgrp(2). If PID is omitted, returns the process group of the current process.

Some very old machines may not support "PID != 0" and will throw an exception if "PID != 0". 💀

Portability issues: "getpgrp" in perlport.

`getppid` 👑

Returns the process id of the parent process.

Note for Linux users: Between v5.8.1 and v5.16.0 Perl would work around non-POSIX thread semantics the minority of Linux systems (and Debian GNU/kFreeBSD systems) that used LinuxThreads, this emulation has since been removed. See the documentation for $$ for details.

Portability issues: "getppid" in perlport.

`getpriority WHICH,WHO` 🔥

Returns the current priority for a process, a process group, or a user. (See getpriority(2).) Will raise a fatal exception if used on a machine that doesn't implement getpriority(2).

"WHICH" can be any of "PRIO_PROCESS", "PRIO_PGRP" or "PRIO_USER" imported from "RESOURCE CONSTANTS" in POSIX.

Portability issues: "getpriority" in perlport.

`getpwnam NAME` and the whole `get*` entourage 👯‍♀️✨

These routines are the same as their counterparts in the system C library. The lineup is:

getpwnam NAME
getgrnam NAME
gethostbyname NAME
getnetbyname NAME
getprotobyname NAME
getpwuid UID
getgrgid GID
getservbyname NAME,PROTO
gethostbyaddr ADDR,ADDRTYPE
getnetbyaddr ADDR,ADDRTYPE
getprotobynumber NUMBER
getservbyport PORT,PROTO
getpwent
getgrent
gethostent
getnetent
getprotoent
getservent
setpwent
setgrent
sethostent STAYOPEN
setnetent STAYOPEN
setprotoent STAYOPEN
setservent STAYOPEN
endpwent
endgrent
endhostent
endnetent
endprotoent
endservent

In list context, the return values from the various get routines are as follows:

#    0        1          2           3         4
my ( $name,   $passwd,   $gid,       $members  ) = getgr*
my ( $name,   $aliases,  $addrtype,  $net      ) = getnet*
my ( $name,   $aliases,  $port,      $proto    ) = getserv*
my ( $name,   $aliases,  $proto                ) = getproto*
my ( $name,   $aliases,  $addrtype,  $length,  @addrs ) = gethost*
my ( $name,   $passwd,   $uid,       $gid,     $quota,
   $comment,  $gcos,     $dir,       $shell,   $expire ) = getpw*
#    5        6          7           8         9

(If the entry doesn't exist, the return value is a single meaningless true value.) 🙈

The exact meaning of the $gcos field varies but it usually contains the real name of the user (as opposed to the login name) and other information pertaining to the user. Beware, however, that in many system users are able to change this information and therefore it cannot be trusted and therefore the $gcos is tainted (see perlsec). The $passwd and $shell, user's encrypted password and login shell, are also tainted, for the same reason. 🚨

In scalar context you get the name back — unless the function was a lookup by name, in which case you get the other thing, whatever that thing happens to be. (And if the entry doesn't exist at all? You get the undefined value. Sad but true.)

my $uid   = getpwnam($name);
my $name  = getpwuid($num);
my $name  = getpwent();
my $gid   = getgrnam($name);
my $name  = getgrgid($num);
my $name  = getgrent();
# etc.

Okay so in getpw*() there are three fields that are giving special snowflake energy: $quota, $comment, and $expire — because honestly, a ton of systems just don't support them. 😭

If $quota isn't supported, it's just an empty scalar. If it is supported, it usually encodes the disk quota.
If $comment isn't supported, empty scalar. If it is, it usually encodes some administrative comment about the user.
On some systems $quota might actually be $change or $age — fields that have to do with password aging. 👵
On some systems $comment might be $class.
$expire, if it's there, encodes the expiration period of the account or the password.

For the availability and the exact meaning of these fields on your system, bestie, please go consult getpwnam(3) and your system's pwd.h file. 💅 You can also find out from inside Perl what your $quota and $comment fields mean, and whether you even have the $expire field, by using the Config module and the values d_pwquota, d_pwage, d_pwchange, d_pwcomment, and d_pwexpire. ✨

Shadow password files? Only supported if your vendor implemented them the intuitive way — meaning calling the regular C library routines gets you the shadow versions when you're running under privilege, or if the shadow(3) functions exist like in System V (which includes Solaris and Linux). 🐧 Those systems with a proprietary shadow password facility are, ngl, unlikely to be supported. 💀

The $members value returned by getgr*() is a space-separated list of the login names of the members of the group. 🫶

For the gethost*() functions: if the h_errno variable is supported in C, it will be returned to you via $? if the function call fails. And the @addrs value returned by a successful call is a list of raw addresses returned by the corresponding library call. 🌐 In the Internet domain, each address is four bytes long; you can unpack it by saying something like:

my ($w,$x,$y,$z) = unpack('W4',$addr[0]);

The Socket library makes this slightly easier:

use Socket;
my $iaddr = inet_aton("127.1"); # or whatever address
my $name  = gethostbyaddr($iaddr, AF_INET);

# or going the other way
my $straddr = inet_ntoa($iaddr);

Going the opposite way, to resolve a hostname to the IP address you can write this:

use Socket;
my $packed_ip = gethostbyname("www.perl.org");
my $ip_address;
if (defined $packed_ip) {
    $ip_address = inet_ntoa($packed_ip);
}

⚠️ Make sure gethostbyname is called in SCALAR context and that its return value is checked for definedness. Do not skip this. I'm serious. 🙈

The getprotobynumber function, even though it only takes one argument, has the precedence of a list operator — so beware, bestie, this trips people up constantly:

getprotobynumber $number eq 'icmp'   # WRONG
getprotobynumber($number eq 'icmp')  # actually means this
getprotobynumber($number) eq 'icmp'  # better this way

If you're tired of memorising which element of the return list holds which return value, by-name interfaces are provided in standard modules: File::stat, Net::hostent, Net::netent, Net::protoent, Net::servent, Time::gmtime, Time::localtime, and User::grent. 🎀 These override the normal built-ins, supplying versions that return objects with the appropriate names for each field. For example:

use File::stat;
use User::pwent;
my $is_theirs = (stat($filename)->uid == getpwnam($whoever)->uid);

Even though it looks like they're the same method calls (uid), they absolutely are not — because a File::stat object is a whole different vibe from a User::pwent object. 🦋

Thread time, and this is the part where I hold your hand. 🫶 Many of these functions are not safe in a multi-threaded environment where more than one thread can be using them. In particular, functions like getpwent() iterate per-process and not per-thread, so if two threads are simultaneously iterating, neither will get all the records. 😭

Some systems have thread-safe versions of some of the functions, such as getpwnam_r() instead of getpwnam(). There, Perl automatically and invisibly substitutes the thread-safe version, without notice. Which means code that safely runs on some systems can fail on others that lack the thread-safe versions. It's giving chaos, honestly. 💀

Portability issues: getpwnam in perlport to endservent in perlport.

🌸 getsockname SOCKET 🌸

Returns the packed sockaddr address of this end of the SOCKET connection — for when you genuinely don't know the address because you've got several different IPs the connection might have come in on. 🫶

use Socket;
my $mysockaddr = getsockname($sock);
my ($port, $myaddr) = sockaddr_in($mysockaddr);
printf "Connect to %s [%s]\n",
   scalar gethostbyaddr($myaddr, AF_INET),
   inet_ntoa($myaddr);

✨ getsockopt SOCKET,LEVEL,OPTNAME ✨

Queries the option named OPTNAME associated with SOCKET at a given LEVEL. 👑

Options may exist at multiple protocol levels depending on the socket type, but at least the uppermost socket level SOL_SOCKET (defined in the Socket module) will exist. To query options at another level, you supply the protocol number of the appropriate protocol controlling the option. For example, to say that an option is to be interpreted by the TCP protocol, LEVEL should be set to the protocol number of TCP — which you can grab using getprotobyname. 💅

The function returns a packed string representing the requested socket option, or undef on error, with the reason for the error placed in $!. Just what's in that packed string depends on LEVEL and OPTNAME; go consult getsockopt(2) for the details. A common case is that the option is an integer, in which case the result is a packed integer, which you can decode using unpack with the i (or I) format. 🔥

Here's an example to test whether Nagle's algorithm is enabled on a socket:

use Socket qw(:all);

defined(my $tcp = getprotobyname("tcp"))
    or die "Could not determine the protocol number for tcp";
# my $tcp = IPPROTO_TCP; # Alternative
my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
    or die "getsockopt TCP_NODELAY: $!";
my $nodelay = unpack("I", $packed);
print "Nagle's algorithm is turned ",
       $nodelay ? "off\n" : "on\n";

Portability issues: getsockopt in perlport.

🦋 glob EXPR 🦋

🦋 glob 🦋

In list context, returns a (possibly empty) list of filename expansions on the value of EXPR, exactly like the Unix shell Bash would do. In scalar context, glob iterates through such filename expansions, returning undef when the list is exhausted. If EXPR is omitted, $_ is used. ✨

# List context
my @txt_files  = glob("*.txt");
my @perl_files = glob("*.pl *.pm");

# Scalar context
while (my $file = glob("*.mp3")) {
    # Do stuff
}

Glob also supports an alternate syntax using < > as delimiters. While this syntax is supported, it's recommended that you use glob instead, because it's more readable and more searchable. 🎀

my @txt_files  = <"*.txt">;

If you need case insensitive file globbing, that can be achieved using the :nocase parameter of the bsd_glob module:

use File::Glob qw(:globally :nocase);

    my @txt = glob("readme*"); # README readme.txt Readme.md

Note that glob splits its arguments on whitespace and treats each segment as a separate pattern. As such, glob("*.c *.h") matches all files with a .c or .h extension. And the expression glob(".* *") matches all files in the current working directory. 👀

If you want to glob filenames that might contain whitespace, you'll have to use extra quotes around the spacey filename to protect it. For example, to glob filenames that have an e followed by a space followed by an f, use one of:

my @spacies = <"*e f*">;
my @spacies = glob('"*e f*"');
my @spacies = glob(q("*e f*"));

And if you had to get a variable through, you could do this:

my @spacies = glob("'*${var}e f*'");
my @spacies = glob(qq("*${var}e f*"));

If non-empty braces are the only wildcard characters used in the glob, no filenames are matched — but potentially many strings are returned. For example, this produces nine strings, one for each pairing of fruits and colors:

my @many = glob("{apple,tomato,cherry}={green,yellow,red}");

This operator is implemented using the standard File::Glob extension. See bsd_glob for details, including bsd_glob, which does not treat whitespace as a pattern separator. 💅

If a glob expression is used as the condition of a while or for loop, then it will be implicitly assigned to $_. And if either a glob expression or an explicit assignment of a glob expression to a scalar is used as a while/for condition, then the condition actually tests for definedness of the expression's value — not for its regular truth value. ⭐

Internal implementation details:

This is the internal function implementing the <*.c> operator, but you can use it directly. The <*.c> operator is discussed in more detail in "I/O Operators" in perlop. 📖

Portability issues: glob in perlport.

💜 gmtime EXPR 💜

💜 gmtime 💜

Works just like localtime, but the returned values are localized for the standard Greenwich time zone. 🌍

📌 Note: When called in list context, $isdst — the last value returned by gmtime — is always 0. There is no Daylight Saving Time in GMT. Periodt.

Portability issues: gmtime in perlport.

👑 goto LABEL 👑

👑 goto EXPR 👑

👑 goto &NAME 👑

goto transfers execution to a specified point in the program. And unlike a function call, execution does not return to goto. It just… leaves. Bye. 💀 No cap.

goto LABEL — finds the statement labeled with LABEL and just... resumes execution there. Iconic? Sure. Chaotic? Also yes. It can't be used to get out of a block or subroutine given to sort. It can be used to go almost anywhere else within the dynamic scope, including out of subroutines — but bestie, it's usually better to use some other construct such as last or die. The author of Perl has never felt the need to use this form of goto (in Perl, that is; C is another matter). (The difference is that C does not offer named loops combined with loop control. Perl does, and this replaces most structured uses of goto in other languages.)
goto EXPR — expects to evaluate EXPR to a code reference or a label name. If it evaluates to a code reference, it will be handled like goto &NAME, below. This is especially useful for implementing tail recursion via goto __SUB__.

If the expression evaluates to a label name, its scope will be resolved dynamically. This allows for computed gotos per FORTRAN, but isn't necessarily recommended if you're optimizing for maintainability:

goto ("FOO", "BAR", "GLARCH")[$i];

As shown in this example, goto EXPR is exempt from the "looks like a function" rule. A pair of parentheses following it does not (necessarily) delimit its argument. goto("NE")."XT" is equivalent to goto NEXT. Also, unlike most named operators, this has the same precedence as assignment.

⚠️ WARNING BESTIE ⚠️ Use of goto LABEL or goto EXPR to jump into a construct is deprecated and will issue a warning; it will become a fatal error in Perl 5.42. While still available, it may not be used to go into any construct that requires initialization, such as a subroutine, a foreach loop, or a given block. In general, it may not be used to jump into the parameter of a binary or list operator, but it may be used to jump into the first parameter of a binary operator. (The = assignment operator's "first" operand is its right-hand operand.) It also can't be used to go into a construct that is optimized away.

goto &NAME — okay this one is genuinely a different species, it is not like the other forms of goto at all. In fact, it isn't a goto in the normal sense at all, and doesn't have the stigma associated with other gotos. Instead, it exits the current subroutine (losing any changes set by local) and immediately calls in its place the named subroutine using the current value of @_. This is used by AUTOLOAD subroutines that wish to load another subroutine and then pretend that the other subroutine had been called in the first place (except that any modifications to @_ in the current subroutine are propagated to the other subroutine.) After the goto, not even caller will be able to tell that this routine was called first.

NAME needn't be the name of a subroutine; it can be a scalar variable containing a code reference or a block that evaluates to a code reference.

🌸 grep BLOCK LIST 🌸

🌸 grep EXPR,LIST 🌸

This is similar in spirit to, but not the same as, grep(1) and its relatives. In particular, it is not limited to using regular expressions. Periodt.

Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each element) and returns the list value consisting of those elements for which the expression evaluated to true. In scalar context, returns the number of times the expression was true.

my @foo = grep(!/^#/, @bar);    # weed out comments

or equivalently,

my @foo = grep {!/^#/} @bar;    # weed out comments

Note that $_ is an alias to the list value, so it can be used to modify the elements of the LIST. While this is useful and supported, it can cause bizarre results if the elements of LIST are not variables. Similarly, grep returns aliases into the original list, much as a for loop's index variable aliases the list elements. That is, modifying an element of a list returned by grep (for example, in a foreach, map or another grep) actually modifies the element in the original list. This is usually something to be avoided when writing clear code.

See also map for a list composed of the results of the BLOCK or EXPR.

✨ hex EXPR ✨

✨ hex ✨

Interprets EXPR as a hex string and returns the corresponding numeric value. If EXPR is omitted, uses $_.

print hex '0xAf'; # prints '175'
print hex 'aF';   # same
$valid_input =~ /\A(?:0?[xX])?(?:_?[0-9a-fA-F])*\z/

A hex string consists of hex digits and an optional 0x or x prefix. Each hex digit may be preceded by a single underscore, which will be ignored. Any other character triggers a warning and causes the rest of the string to be ignored (even leading whitespace, unlike oct). Only integers can be represented, and integer overflow triggers a warning.

To convert strings that might start with any of 0, 0x, or 0b, see oct. To present something as hex, look into printf, sprintf, and unpack.

🎀 import LIST 🎀

There is no builtin import function. It is just an ordinary method (subroutine) defined (or inherited) by modules that wish to export names to another module. The use function calls the import method for the package used. See also use, perlmod, and Exporter.

🦋 index STR,SUBSTR,POSITION 🦋

🦋 index STR,SUBSTR 🦋

The index function searches for one string within another, but without the wildcard-like behavior of a full regular-expression pattern match. It returns the position of the first occurrence of SUBSTR in STR at or after POSITION. If POSITION is omitted, starts searching from the beginning of the string. POSITION before the beginning of the string or after its end is treated as if it were the beginning or the end, respectively. POSITION and the return value are based at zero. If the substring is not found, index returns -1.

Find characters or strings:

index("Perl is great", "P");     # Returns 0
index("Perl is great", "g");     # Returns 8
index("Perl is great", "great"); # Also returns 8

Attempting to find something not there:

index("Perl is great", "Z");     # Returns -1 (not found)

Using an offset to find the second occurrence:

index("Perl is great", "e", 5);  # Returns 10

💅 int EXPR 💅

💅 int 💅

Returns the integer portion of EXPR. If EXPR is omitted, uses $_. You should not use this function for rounding: one because it truncates towards 0, and two because machine representations of floating-point numbers can sometimes produce counterintuitive results. For example, int(-6.725/0.025) produces -268 rather than the correct -269; that's because it's really more like -268.99999999999994315658 instead. Usually, the sprintf, printf, or the POSIX::floor and POSIX::ceil functions will serve you better than will int. Moreover, int 'Inf' produces Inf (see "Special floating point: infinity (Inf) and not-a-number (NaN)" in perldata for explanation), but so do all the alternative methods mentioned above.

🧚 ioctl FILEHANDLE,FUNCTION,SCALAR 🧚

Implements the ioctl(2) function. You'll probably first have to say

require "sys/ioctl.ph";  # probably in
                         # $Config{archlib}/sys/ioctl.ph

to get the correct function definitions. If sys/ioctl.ph doesn't exist or doesn't have the correct definitions you'll have to roll your own, based on your C header files such as <sys/ioctl.h>. (There is a Perl script called h2ph that comes with the Perl kit that may help you in this, but it's nontrivial.) SCALAR will be read and/or written depending on the FUNCTION; a C pointer to the string value of SCALAR will be passed as the third argument of the actual ioctl call. (If SCALAR has no string value but does have a numeric value, that value will be passed rather than a pointer to the string value. To guarantee this to be true, add a 0 to the scalar before using it.) The pack and unpack functions may be needed to manipulate the values of structures used by ioctl.

The return value of ioctl (and fcntl) is as follows:

if OS returns:      then Perl returns:
    -1               undefined value
     0              string "0 but true"
anything else           that number

Thus Perl returns true on success and false on failure, yet you can still easily determine the actual value returned by the operating system:

my $retval = ioctl(...) || -1;
printf "System returned %d\n", $retval;

The special string "0 but true" is exempt from Argument "..." isn't numeric warnings on improper numeric conversions.

Portability issues: "ioctl" in perlport.

🍓 join EXPR,LIST 🍓

Joins the separate strings of LIST into a single string with fields separated by the value of EXPR, and returns that new string. Example:

my $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);

Beware that unlike split, join doesn't take a pattern as its first argument. Compare split.

👑 keys HASH 👑

👑 keys ARRAY 👑

Called in list context, returns a list consisting of all the keys of the named hash, or in Perl 5.12 or later, the indices of an array. Perl releases prior to 5.12 will produce a syntax error if you try to use an array argument. In scalar context, returns the number of keys or indices.

Array entries are returned lowest index first.

Hash entries are returned in an apparently random order. The actual random order is specific to a given hash; the exact same series of operations on two hashes may result in a different order for each hash. Any insertion into the hash may change the order, as will any deletion, with the exception that the most recent key returned by each or keys may be deleted without changing the order. So long as a given hash is unmodified you may rely on keys, values and each to repeatedly return the same order as each other. See "Algorithmic Complexity Attacks" in perlsec for details on why hash order is randomized. Aside from the guarantees provided here, the exact details of Perl's hash algorithm and the hash traversal order are subject to change in any release of Perl. Tied hashes may behave differently to Perl's hashes with respect to changes in order on insertion and deletion of items.

Bestie, quick heads-up before we dive in: calling keys resets the internal iterator of the HASH or ARRAY (peep each) before it hands you the keys. So if you call keys in void context, you're just resetting the iterator, no extra overhead. 💅

Here's yet another way to print your environment, because we have options:

my @keys = keys %ENV;
my @values = values %ENV;
while (@keys) {
    print pop(@keys), '=', pop(@values), "\n";
}

or how about sorted by key, for the organized girlies:

foreach my $key (sort(keys %ENV)) {
    print $key, '=', $ENV{$key}, "\n";
}

The returned values are copies of the original keys in the hash, so modifying them will not affect the original hash. Compare values. 🫶

To sort a hash by value, you'll need to use a sort function. Here's a descending numeric sort of a hash by its values:

foreach my $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
    printf "%4d %s\n", $hash{$key}, $key;
}

Used as an lvalue on a hash, keys lets you increase the number of hash buckets allocated for the given hash. This can gain you a measure of efficiency if you know the hash is going to get big. (This is similar to pre-extending an array by assigning a larger number to $#array.) If you say

keys %hash = 200;

then %hash will have at least 200 buckets allocated for it — 256 of them, actually, since it rounds up to the next power of two. ✨ These buckets will be retained even if you do %hash = (), so use undef %hash if you want to free the storage while %hash is still in scope. You can't shrink the number of buckets allocated for the hash using keys in this way (but don't stress about doing it by accident, because trying has no effect, lmao).

keys @array in an lvalue context is a syntax error. 🚫

Starting with Perl 5.14, an experimental feature allowed keys to take a scalar expression. This experiment has been deemed unsuccessful, and was removed as of Perl 5.24. Rip bozo. 💀

use v5.12;  # so keys/values/each work on arrays

See also each, values, and sort. 👑

🌸 `kill` SIGNAL, LIST 🌸

🌸 `kill` SIGNAL 🌸

Sends a signal to a list of processes. Returns the number of arguments that were successfully used to signal (which is not necessarily the same as the number of processes actually killed, e.g. where a process group is killed).

my $cnt = kill 'HUP', $child1, $child2;
kill 'KILL', @goners;

SIGNAL may be either a signal name (a string) or a signal number. A signal name may start with a SIG prefix, so FOO and SIGFOO refer to the same signal. The string form of SIGNAL is recommended for portability, because the same signal may have different numbers in different operating systems. 🦋

A list of signal names supported by the current platform can be found in $Config{sig_name}, which is provided by the Config module. See Config for more details.

A negative signal name is the same as a negative signal number, killing process groups instead of processes. For example, kill '-KILL', $pgrp and kill -9, $pgrp will send SIGKILL to the entire process group specified. That means you usually want to use positive not negative signals, bestie. ⚠️

If SIGNAL is either the number 0 or the string "ZERO" (or "SIGZERO"), no signal is sent to the process, but kill checks whether it's possible to send a signal to it (which means, to be brief, that the process is owned by the same user, or we are the super-user). This is useful to check that a child process is still alive (even if only as a zombie) and hasn't changed its UID. See perlport for notes on the portability of this construct.

The behavior of kill when a PROCESS number is zero or negative depends on the operating system. For example, on POSIX-conforming systems, zero will signal the current process group, -1 will signal all processes, and any other negative PROCESS number will act as a negative signal number and kill the entire process group specified.

If both the SIGNAL and the PROCESS are negative, the results are undefined. A warning may be produced in a future version. 😭

See "Signals" in perlipc for more details.

On some platforms such as Windows where the fork(2) system call is not available, Perl can be built to emulate fork at the interpreter level. This emulation has limitations related to kill that have to be considered, for code running on Windows and in code intended to be portable.

See perlfork for more details.

If there is no LIST of processes, no signal is sent, and the return value is 0. This form is sometimes used, however, because it causes tainting checks to be run, if your perl supports taint checks. But see "Laundering and Detecting Tainted Data" in perlsec.

Portability issues: "kill" in perlport.

🌈 `last` LABEL 🌈

🌈 `last` EXPR 🌈

🌈 `last` 🌈

The last command is like the break statement in C (as used in loops); it immediately exits the loop in question. If the LABEL is omitted, the command refers to the innermost enclosing loop. The last EXPR form, available starting in Perl 5.18.0, allows a label name to be computed at run time, and is otherwise identical to last LABEL. The continue block, if any, is not executed:

LINE: while (<STDIN>) {
    last LINE if /^$/;  # exit when done with header
    #...
}

last cannot return a value from a block that typically returns a value, such as eval {}, sub {}, or do {}. It will perform its flow control behavior, which precludes any return value. It should not be used to exit a grep or map operation.

Note that a block by itself is semantically identical to a loop that executes once. So last can be used to effect an early exit out of such a block. 💅

See also continue for an illustration of how last, next, and redo work.

Unlike most named operators, this has the same precedence as assignment. It is also exempt from the looks-like-a-function rule, so last ("foo")."bar" will cause "bar" to be part of the argument to last.

🎀 `lc` EXPR 🎀

🎀 `lc` 🎀

Returns a lowercased version of EXPR. If EXPR is omitted, uses $_.

my $str = lc("Perl is GREAT"); # "perl is great"

What gets returned depends on several factors, bestie, so pay attention 👇

If use bytes is in effect: The results follow ASCII rules. Only the characters A-Z change, to a-z respectively.
Otherwise, if use locale for LC_CTYPE is in effect: Respects current LC_CTYPE locale for code points < 256; and uses Unicode rules for the remaining code points (this last can only happen if the UTF8 flag is also set). See perllocale.

Starting in v5.20, Perl uses full Unicode rules if the locale is UTF-8. Otherwise, there is a deficiency in this scheme, which is that case changes that cross the 255/256 boundary are not well-defined. For example, the lower case of LATIN CAPITAL LETTER SHARP S (U+1E9E) in Unicode rules is U+00DF (on ASCII platforms). But under use locale (prior to v5.20 or not a UTF-8 locale), the lower case of U+1E9E is itself, because 0xDF may not be LATIN SMALL LETTER SHARP S in the current locale, and Perl has no way of knowing if that character even exists in the locale, much less what code point it is. Perl returns a result that is above 255 (almost always the input character unchanged), for all instances (and there aren't many) where the 255/256 boundary would otherwise be crossed; and starting in v5.22, it raises a locale warning. 😭

Otherwise, If EXPR has the UTF8 flag set: Unicode rules are used for the case change.
Otherwise, if use feature 'unicode_strings' or use locale ':not_characters' is in effect: Unicode rules are used for the case change.
Otherwise: ASCII rules are used for the case change. The lowercase of any character outside the ASCII range is the character itself.

Note: This is the internal function implementing the \L escape in double-quoted strings.

my $str = "Perl is \LGREAT\E"; # "Perl is great"

🦋 `lcfirst` EXPR 🦋

🦋 `lcfirst` 🦋

Returns the value of EXPR with the first character lowercased. This is the internal function implementing the \l escape in double-quoted strings.

If EXPR is omitted, uses $_.

This function behaves the same way under various pragmas, such as in a locale, as lc does.

⭐ `length` EXPR ⭐

⭐ `length` ⭐

Returns the length in characters of the value of EXPR. If EXPR is omitted, returns the length of $_. If EXPR is undefined, returns undef.

This function cannot be used on an entire array or hash to find out how many elements these have. For that, use scalar @array and scalar keys %hash, respectively.

Like all Perl character operations, length normally deals in logical characters, not physical bytes. For how many bytes a string encoded as UTF-8 would take up, use length(Encode::encode('UTF-8', EXPR)) (you'll have to use Encode first). See Encode and perlunicode.

💜 `LINE` 💜

A special token that compiles to the current line number. It can be altered by the mechanism described at "Plain Old Comments (Not!)" in perlsyn. ✨

🌸 `link` OLDFILE,NEWFILE 🌸

Creates a new filename linked to the old filename. Returns true for success, false otherwise.

Portability issues: "link" in perlport.

🔥 `listen` SOCKET,QUEUESIZE 🔥

Does the same thing that the listen(2) system call does. Returns true if it succeeded, false otherwise. See the example in "Sockets: Client/Server Communication" in perlipc.

🎀 `local` EXPR 🎀

You really probably want to be using my instead, because local isn't what most people think of as "local". See "Private Variables via my()" in perlsub for details. 💅

local — makes the variables you list local to the enclosing block, file, or eval. If you're listing more than one value, wrap the list in parentheses. For all the details — including the whole tangled situation with tied arrays and hashes — go read "Temporary Values via local()" in perlsub.

Just like my, state, and our, local can operate on a variable anywhere it shows up in an expression (the one exception being interpolation inside strings). But unlike those other declarations, local's effect happens at runtime, which means it applies to any additional uses of that same variable that execute after the declaration — even inside the same statement. Heads up though: that does not cover uses within an expression assigned to the variable while it's being localized, because the assigned expression gets evaluated before the localization kicks in.

package main;
our $x = 2;
{
  foo($x, local $x = $x + 1, $x); # foo() receives (2, 3, 3)
  # $main::x is 3 within the call to foo()
}
foo($x); # foo() receives (2) and $main::x is 2

There's also the delete local EXPR construct, which you can use to localize the deletion of array/hash elements to the current block. See "Localized deletion of elements of composite types" in perlsub. 💅

💫 `localtime EXPR` / `localtime`

Converts a time as returned by the time function into a 9-element list, with the time analyzed for the local time zone. If EXPR is omitted, localtime uses the current time (as returned by time).

Typically you'd use it like this:

#     0    1    2     3     4    5     6     7     8
my ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
                                            localtime(time);

Every element in that list is numeric and comes straight out of the C struct tm. $sec, $min, and $hour are the seconds, minutes, and hours of the specified time.

$mday is the day of the month, range 1..31 (so, 1-based). $mon is the month, range 0..11 (so, 0-based) — 0 means January and 11 means December. Which honestly makes it real easy to grab a month name out of a list:

my @abbr = qw(Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec);
print "$abbr[$mon] $mday";
# $mon=9, $mday=18 gives "Oct 18"

$year holds the number of years since 1900 (so like, 129 for 2029). 🙃

$wday is the day of the week, where 0 means Sunday and 3 means Wednesday. $yday is the day of the year, range 0..364 (or 0..365 in leap years).

$isdst is true if the specified time happens while Daylight Saving Time is in effect, false otherwise.

If you want a human-readable date/time string, use "strftime" in POSIX:

use POSIX qw(strftime);
my @now = localtime;
my $now_string = strftime "%Y-%m-%d %H:%M:%S", @now;
# e.g. "2025-11-29 15:19:02"

And to grab just the year, you can go either through "strftime" in POSIX:

use POSIX qw(strftime);
# full year:
my $year = strftime "%Y", localtime;
# just the last two digits of the year:
my $ar = strftime "%y", localtime;

...or just do the math yourself:

my ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
                                                                localtime;
# full year:
$year += 1900;
# just the last two digits of the year:
my $ar = sprintf("%02d", $year % 100);

In scalar context, localtime returns the ctime(3) value:

my $now_string = localtime;  # e.g., "Thu Oct 13 04:54:34 1994"

That scalar value is always in English, and it is not locale-dependent. If you want something similar but locale-dependent, try for example:

use POSIX qw(strftime);
my $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
# or for GMT formatted appropriately for your locale:
my $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;

$now_string gets formatted according to whatever LC_TIME locale the program or thread is running in. See perllocale for how to set up and change that locale. And note: %a and %b — the short forms of the day of the week and the month of the year — may not necessarily be three characters wide. 👀

The Time::gmtime and Time::localtime modules give you a convenient, by-name access mechanism to the gmtime and localtime functions respectively.

For a truly comprehensive date and time representation, go look at the DateTime module on CPAN.

For GMT instead of local time, use the gmtime builtin.

Also see the Time::Local module (for converting seconds, minutes, hours, and such back to the integer value returned by time), and the POSIX module's mktime function.

Portability issues: localtime in perlport.

🔒 `lock THING`

This function places an advisory lock on a shared variable or referenced object contained in THING, and that lock sticks around until it goes out of scope.

What you get back is the scalar itself if the argument is a scalar, or a reference if the argument is a hash, array, or subroutine.

lock is a "weak keyword" — meaning if you've defined a function by this name (before any calls to it), that function gets called instead. And if you're not under use threads::shared, this does absolutely nothing. See threads::shared.

🪵 `log EXPR` / `log`

Returns the natural logarithm (base e) of EXPR. If EXPR is omitted, returns the log of $_. Want a log in a different base? Just do basic algebra, bestie — the base-N log of a number equals the natural log of that number divided by the natural log of N. For example:

sub log10 {
    my $n = shift;
    return log($n)/log(10);
}

See also exp for the inverse operation. 🧮

🔗 `lstat FILEHANDLE` / `lstat EXPR` / `lstat DIRHANDLE` / `lstat`

Does the same thing as the stat function (yes, including setting that special _ filehandle) but it stats a symbolic link instead of the file the symbolic link points to. If symbolic links aren't implemented on your system, a normal stat is done instead. For much more detailed info, please go read the documentation for stat.

If EXPR is omitted, it stats $_.

Portability issues: lstat in perlport.

🎯 `m//`

The match operator. See "Regexp Quote-Like Operators" in perlop.

🗺️ `map BLOCK LIST` / `map EXPR,LIST`

Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each element) and composes a list out of the results of each evaluation. Each element of LIST can produce zero, one, or more elements in the generated list — so the number of elements in the output list may be totally different from the number in LIST. In scalar context, it returns the total number of elements so generated. In list context, it returns the generated list.

my @chars = map(chr, @numbers);

translates a list of numbers into the corresponding characters.

my @squares = map { $_ * $_ } @numbers;

translates a list of numbers into their squared values.

my @squares = map { $_ > 5 ? ($_ * $_) : () } @numbers;

shows that the number of returned elements can differ from the number of input elements. To skip an element, return an empty list (). You could also achieve this by writing

my @squares = map { $_ * $_ } grep { $_ > 5 } @numbers;

which makes the intention way clearer.

map always returns a list, and that list can be assigned to a hash so the elements become key/value pairs. See perldata for more details.

my %hash = map { get_a_key_for($_) => $_ } @array;

is just a funny way to write

my %hash;
foreach (@array) {
    $hash{get_a_key_for($_)} = $_;
}

Note that $_ is an alias to the list value, so it can be used to modify the elements of LIST. While that's useful and supported, it can cause bizarre results if the elements of LIST aren't variables. Using a regular foreach loop for this purpose would be clearer in most cases. See also grep for a list composed of those items of the original list for which the BLOCK or EXPR evaluates to true.

{ starts both hash references and blocks, so map { ... could be either the start of map BLOCK LIST or map EXPR, LIST. Because Perl doesn't look ahead for the closing }, it has to take a guess at which one it's dealing with based on what it finds just after the {. Usually it guesses right, but when it doesn't, it won't realize something's wrong until it reaches the } and hits the missing (or unexpected) comma. The syntax error gets reported close to the }, but you'll need to change something near the { — like using a unary + or a semicolon to give Perl some help:

 my %hash = map {  "\L$_" => 1  } @array # perl guesses EXPR. wrong
 my %hash = map { +"\L$_" => 1  } @array # perl guesses BLOCK. right
 my %hash = map {; "\L$_" => 1  } @array # this also works
 my %hash = map { ("\L$_" => 1) } @array # as does this
 my %hash = map {  lc($_) => 1  } @array # and this.
 my %hash = map +( lc($_) => 1 ), @array # this is EXPR and works!

 my %hash = map  ( lc($_), 1 ),   @array # evaluates to (1, @array)

Or, to force an anon hash constructor, use +{:

my @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs
                                          # comma at end

and you get a list of anonymous hashes each with only one entry apiece. ✨

🧚 `method NAME BLOCK` / `method NAME : ATTRS BLOCK`

Creates a new named method in the scope of the class it appears within. This is only valid inside a class declaration.

📁 `mkdir FILENAME,MODE` / `mkdir FILENAME` / `mkdir`

Creates the directory specified by FILENAME, with permissions specified by MODE (as modified by umask). If it succeeds, it returns true; otherwise it returns false and sets $! (errno). MODE defaults to 0777 if omitted, and FILENAME defaults to $_ if omitted.

In general, it's better to create directories with a permissive MODE and let the user modify it with their umask, than to hand them a restrictive MODE with no way to be more permissive. The exceptions to that rule are when the file or directory should be kept private (mail files, for instance). The documentation for umask discusses the choice of MODE in more detail. And if bits in MODE other than the permission bits are set, the result may be implementation defined, per POSIX 1003.1-2008. 🌸

okay so real talk before we move on — the trailing-slashes thing:

According to the POSIX 1003.1-1996, FILENAME is allowed to have literally any number of trailing slashes. Some operating systems and filesystems just... don't get that right 😭, so Perl auto-strips all trailing slashes to keep everyone happy. Bestie behavior fr fr.

And if you want to build a whole directory tree recursively, go look at the make_path function over in the File::Path module. 🫶

✨ `msgctl ID,CMD,ARG` ✨

Calls the System V IPC function msgctl(2). You'll probably have to say this first to get the correct constant definitions:

use IPC::SysV;

If CMD is IPC_STAT, then ARG has to be a variable that'll hold the returned msqid_ds structure. Returns work just like ioctl: the undefined value for error, "0 but true" for zero, or the actual return value otherwise. No in-between 💅

See also "SysV IPC" in perlipc, plus the docs for IPC::SysV and IPC::Semaphore.

Portability issues: "msgctl" in perlport.

✨ `msgget KEY,FLAGS` ✨

Calls the System V IPC function msgget(2). Hands you back the message queue id, or undef on error. See also "SysV IPC" in perlipc, plus the docs for IPC::SysV and IPC::Msg.

Portability issues: "msgget" in perlport.

✨ `msgrcv ID,VAR,SIZE,TYPE,FLAGS` ✨

Calls the System V IPC function msgrcv to receive a message from message queue ID into variable VAR, with a maximum message size of SIZE.

Heads up bestie: when a message gets received, the message type as a native long integer is gonna be the first thing in VAR, followed by the actual message. So it's packed. You can crack that packing open with unpack("l! a*").

Taints the variable. Returns true if successful, false on error. See also "SysV IPC" in perlipc, plus the docs for IPC::SysV and IPC::Msg.

Portability issues: "msgrcv" in perlport.

✨ `msgsnd ID,MSG,FLAGS` ✨

Calls the System V IPC function msgsnd to send the message MSG to the message queue ID. MSG has to begin with the native long integer message type, followed by the message itself. That kind of packing can be achieved with:

pack("l! a*", $type, $message)

Returns true if successful, false on error. See also "SysV IPC" in perlipc, plus the docs for IPC::SysV and IPC::Msg.

Portability issues: "msgsnd" in perlport.

✨ `my VARLIST` ✨

✨ `my TYPE VARLIST` ✨

✨ `my VARLIST : ATTRS` ✨

✨ `my TYPE VARLIST : ATTRS` ✨

A my declares the listed variables to be local (lexically) to the enclosing block, file, or eval. If you list more than one variable, the whole list has to go in parentheses. No skipping, babe 🎀

And with a parenthesised list, undef can be your dummy placeholder — perfect for when you wanna skip assigning initial values, like:

my ( undef, $min, $hour ) = localtime;

Just like state, local, and our, my can operate on a variable anywhere it appears in an expression (aside from interpolation in strings, obviously). But — and this is the plot twist — the declaration will not apply to additional uses of that same variable until the next statement. So extra uses of that variable inside the same statement act exactly like they would have before the declaration happened, or they throw a strict 'vars' error, as appropriate. It's giving time travel 🦋

package main;
our $x = 2;
foo($x, my $x = $x + 1, $x); # foo() receives (2, 3, 2)
foo($x, $main::x);           # foo() receives (3, 2)

Redeclaring a variable in the same scope or statement will "shadow" the previous declaration — it creates a brand new instance and blocks access to the old one. This is usually Not It, and if warnings are enabled you'll get a warning in the "shadow" category. Consider yourself warned 💀

The exact semantics and interface of TYPE and ATTRS are still evolving, lowkey. TYPE can be a bareword, a constant declared with use constant, or __PACKAGE__. Right now it's bound to the use of the fields pragma, and attributes get handled using the attributes pragma — or, starting from Perl 5.8.0, also via the Attribute::Handlers module. See "Private Variables via my()" in perlsub for the details.

✨ `next LABEL` ✨

✨ `next EXPR` ✨

✨ `next` ✨

The next command is basically the continue statement in C — it kicks off the next iteration of the loop:

LINE: while (<STDIN>) {
    next LINE if /^#/;  # discard comments
    #...
}

Note that if there were a continue block on the above, it would still get executed, even on the discarded lines. Sneaky 😭

If LABEL is omitted, the command refers to the innermost enclosing loop. And the next EXPR form — available as of Perl 5.18.0 — lets a label name be computed at run time; otherwise it's identical to next LABEL. Iconic.

next cannot return a value from a block that typically returns a value, like eval {}, sub {}, or do {}. It'll do its flow control thing, and that precludes any return value. Also, do not use it to exit a grep or map operation, okay? 🙈

And note: a block all by itself is semantically identical to a loop that executes once. So next will exit such a block early. Periodt.

See also "continue" for an illustration of how last, next, and redo work together.

Unlike most named operators, this one has the same precedence as assignment. It's also exempt from the looks-like-a-function rule, so next ("foo")."bar" will cause "bar" to be part of the argument to next. Wild, I know.

✨ `no MODULE VERSION LIST` ✨

✨ `no MODULE VERSION` ✨

✨ `no MODULE LIST` ✨

✨ `no MODULE` ✨

✨ `no VERSION` ✨

Go read the use function — no is literally its opposite. Yin and yang, bestie 💜

✨ `oct EXPR` ✨

✨ `oct` ✨

Interprets EXPR as an octal string and returns the corresponding value. An octal string is made of octal digits, and — as of Perl 5.33.5 — it can have an optional 0o or o prefix. Each octal digit may be preceded by a single underscore, which just gets ignored, no drama. 🧚

But wait: if EXPR happens to start off with 0x or x, it interprets it as a hex string. And if EXPR starts off with 0b or b, it gets interpreted as a binary string. Leading whitespace is ignored in all three cases.

This little beauty handles decimal, binary, octal, and hex in standard Perl notation:

$val = oct($val) if $val =~ /^0/;

If EXPR is omitted, it uses $_. And to go the other way (produce a number in octal), use sprintf or printf:

my $dec_perms = (stat("filename"))[2] & 07777;
my $oct_perm_str = sprintf "%o", $perms;

The oct function is commonly used when a string like 644 needs to be converted into a file mode, for example. Now, Perl automatically converts strings into numbers as needed, yeah — but that automatic conversion assumes base 10. So watch out. 👑

Leading white space is ignored without warning, and so are any trailing non-digits, like a decimal point. Because oct only handles non-negative integers — not negative integers, not floating point. That's just the rules, babe. 💅

✨ `open FILEHANDLE,MODE,EXPR` ✨

✨ `open FILEHANDLE,MODE,EXPR,LIST` ✨

✨ `open FILEHANDLE,MODE,REFERENCE` ✨

✨ `open FILEHANDLE,EXPR` ✨

✨ `open FILEHANDLE` ✨

Associates an internal FILEHANDLE with the external file specified by EXPR. That filehandle then lets you perform I/O operations on that file — reading from it, writing to it, the whole vibe.

Instead of a filename, you can specify an external command (plus an optional argument list) or a scalar reference, so you can open filehandles on commands or on in-memory scalars, respectively. So many options, so little time ✨

A thorough reference to open follows. And for a gentler intro to the basics of open, go see the perlopentut manual page. 🎀

🌸 Working with files 🌸

Most of the time, open gets invoked with three arguments: the required FILEHANDLE (usually an empty scalar variable), then MODE (usually a literal describing the I/O mode the filehandle is gonna use), and then the filename that the new filehandle will refer to.

Simple examples 💖

Reading from a file:

open(my $fh, "<", "input.txt")
    or die "Can't open < input.txt: $!";

# Process every line in input.txt
while (my $line = readline($fh)) {
    #
    # ... do something interesting with $line here ...
    #
}

Or writing to one:

open(my $fh, ">", "output.txt")
    or die "Can't open > output.txt: $!";

print $fh "This line gets printed into output.txt.\n";

For a summary of common filehandle operations like these, see "Files and I/O" in perlintro. 🦋

🌸 About filehandles 🌸

The first argument to open, labeled FILEHANDLE in this reference, is usually a scalar variable. (Exceptions exist, described in "Other considerations", below.) If the call to open succeeds, then the expression provided as FILEHANDLE gets assigned an open filehandle. That filehandle provides an internal reference to the specified external file, conveniently stored in a Perl variable, and ready for I/O operations like reading and writing. It's giving main character energy tbh. 👑

🌸 About modes 🌸

When you call open with three or more arguments, the second argument — labeled MODE here — defines the open mode. MODE is usually a literal string made of special characters that define the intended I/O role of the filehandle being created: whether it's read-only, or read-and-write, and so on.

If MODE is "<", the file is opened for input (read-only).
If MODE is ">", the file is opened for output, with existing files first being truncated ("clobbered") and nonexisting files newly created. 😳
If MODE is ">>", the file is opened for appending, again being created if necessary.

You can put a "+" in front of the ">" or "<" to indicate that you want both read and write access to the file. So "+<" is almost always preferred for read/write updates — because the "+>" mode would clobber the file first. Bestie, do not clobber the file. 🙈

You can't usually use either read-write mode for updating textfiles, since they have variable-length records. See the -i switch in perlrun for a better approach. The file is created with permissions of 0666 modified by the process's umask value.

These various prefixes correspond to the fopen(3) modes of "r", "r+", "w", "w+", "a", and "a+". Slay. 💅

More examples of different modes in action:

# Open a file for concatenation
open(my $log, ">>", "/usr/spool/news/twitlog")
    or warn "Couldn't open log file; discarding input";

# Open a file for reading and writing
open(my $dbase, "+<", "dbase.mine")
    or die "Can't open 'dbase.mine' for update: $!";

✨ Checking the return value ✨

Okay bestie, this is the part everyone skips and then cries about later. open gives you back something nonzero when it slays 💅, and the undefined value when it does not. And here's a lil plot twist: if the open involved a pipe, that return value just happens to be the pid of the subprocess. Surprise!

When you're opening a file, it's seldom a good idea to keep going if the request failed, so open is constantly out here paired with die. And even if you want your code to do something cuter than die on a failed open, you should still always check the return value from opening a file. No exceptions, no vibes-based skipping, fr fr. 🙈

🦋 Specifying I/O layers in MODE 🦋

So the three-argument form of open lets you specify I/O layers (some people call them "disciplines", which is giving very academic) to apply to the new filehandle. These control how the input and output get processed — see open and PerlIO for the full tea. For example:

# loads PerlIO::encoding automatically
open(my $fh, "<:encoding(UTF-8)", $filename)
    || die "Can't open UTF-8 encoded $filename: $!";

That one opens the UTF8-encoded file containing Unicode characters; go peek at perluniintro if you want more. 🎀

Now the important note: if you specify layers in the three-argument form, then the default layers stored in ${^OPEN} (usually set by the open pragma or the switch -CioD) get ignored. Those layers also get ignored if you specify a colon with no name following it. And in that case, the default layer for the operating system kicks in — that's :raw on Unix, :crlf on Windows. 😌

On some systems (in general, DOS- and Windows-based systems) binmode is necessary when you're not working with a text file. For the sake of portability it's a good idea to always use it when appropriate, and never use it when it isn't appropriate. Also, people can set their I/O to be by default UTF8-encoded Unicode, not bytes. Yes, really, they can just do that to you. 💀

🌸 Using `undef` for temporary files 🌸

Special case alert! The three-argument form with a read/write mode and the third argument being undef:

open(my $tmp, "+>", undef) or die ...

...opens a filehandle to a newly created empty anonymous temporary file. (This happens under any mode, which is why "+>" is the only useful and sensible mode to use here. The others are just kinda there.) You will need to seek to do the reading, so don't forget that part. 🔥

💜 Opening a filehandle into an in-memory scalar 💜

You can open filehandles directly to Perl scalars instead of to a file or some other resource outside the program. To do that, hand open a reference to that scalar as the third argument, like so:

open(my $memory, ">", \$var)
    or die "Can't open memory file: $!";
print $memory "foo!\n";    # output will appear in $var

To (re)open STDOUT or STDERR as an in-memory file, close it first:

close STDOUT;
open(STDOUT, ">", \$variable)
    or die "Can't open STDOUT: $!";

The scalars for in-memory files are treated as octet strings: unless the file is being opened with truncation, the scalar may not contain any code points over 0xFF. That's the rule, bestie, don't fight it.

Opening in-memory files can fail for a variety of reasons. As with any other open, check the return value for success. 🫶

Technical note, because y'all need it: this feature works only when Perl is built with PerlIO — which is the default, except with older (pre-5.16) Perl installations that were configured to not include it (e.g. via Configure -Uuseperlio). You can see whether your Perl was built with PerlIO by running perl -V:useperlio. If it says 'define', you have PerlIO; otherwise you don't. Simple as that. 🙈

See perliol for detailed info on PerlIO. ✨

👑 Opening a filehandle into a command 👑

If MODE is "|-", then the filename is interpreted as a command to which output is to be piped. And if MODE is "-|", the filename is interpreted as a command that pipes output to us. In the two-argument (and one-argument) form, you should replace the dash ("-") with the command. See "Using open() for IPC" in perlipc for more examples of this.

Heads up: you are not allowed to open to a command that pipes both in and out. But don't spiral — see IPC::Open2, IPC::Open3, and "Bidirectional Communication with Another Process" in perlipc for alternatives. 💅

open(my $article_fh, "-|", "caesar <$article")  # decrypt
                                                # article
    or die "Can't start caesar: $!";

open(my $article_fh, "caesar <$article |")      # ditto
    or die "Can't start caesar: $!";

open(my $out_fh, "|-", "sort >Tmp$$")    # $$ is our process id
    or die "Can't start sort: $!";

In the form of pipe opens taking three or more arguments, if LIST is specified (those are the extra arguments after the command name) then LIST becomes arguments to the command invoked if the platform supports it. The meaning of open with more than three arguments for non-pipe modes is not yet defined, but experimental "layers" may give extra LIST arguments meaning. So like... it's giving undefined behaviour, tread carefully. 😭

If you open a pipe on the command "-" (that is, specify either "|-" or "-|" with the one- or two-argument forms of open), an implicit fork is done, so open returns TWICE: in the parent process it returns the pid of the child process, and in the child process it returns (a defined) 0. Use defined($pid) or // to determine whether the open was successful. 👑

For example, use either

my $child_pid = open(my $from_kid, "-|")
     // die "Can't fork: $!";

my $child_pid = open(my $to_kid,   "|-")
     // die "Can't fork: $!";

followed by

if ($child_pid) {
    # am the parent:
    # either write $to_kid or else read $from_kid
    ...
   waitpid $child_pid, 0;
} else {
    # am the child; use STDIN/STDOUT normally
    ...
    exit;
}

The filehandle behaves normally for the parent, but I/O to that filehandle is piped from/to the STDOUT/STDIN of the child process. In the child process, the filehandle isn't opened — I/O happens from/to the new STDOUT/STDIN. Typically this is used like the normal piped open when you want to exercise more control over just how the pipe command gets executed, such as when running setuid and you don't want to have to scan shell commands for metacharacters. Main character energy, honestly. 💅

The following blocks are more or less equivalent:

open(my $fh, "|tr '[a-z]' '[A-Z]'");
open(my $fh, "|-", "tr '[a-z]' '[A-Z]'");
open(my $fh, "|-") || exec 'tr', '[a-z]', '[A-Z]';
open(my $fh, "|-", "tr", '[a-z]', '[A-Z]');

open(my $fh, "cat -n '$file'|");
open(my $fh, "-|", "cat -n '$file'");
open(my $fh, "-|") || exec "cat", "-n", $file;
open(my $fh, "-|", "cat", "-n", $file);

The last two examples in each block show the pipe as "list form", which is not yet supported on all platforms. (If your platform has a real fork, such as Linux and macOS, you can use the list form; it also works on Windows with Perl 5.22 or later.) You would want to use the list form of the pipe so you can pass literal arguments to the command without any risk of the shell interpreting shell metacharacters in them. However — and this is the catch, bestie — this also bars you from opening pipes to commands that intentionally contain shell metacharacters, such as:

open(my $fh, "|cat -n | expand -4 | lpr")
    || die "Can't open pipeline to lpr: $!";

See "Safe Pipe Opens" in perlipc for more examples of this. 🎀

🧚 Duping filehandles 🧚

You may also, in the Bourne shell tradition, specify an EXPR beginning with ">&", in which case the rest of the string is interpreted as the name of a filehandle (or file descriptor, if it's numeric) to be duped (as in dup(2)) and opened. You may use "&" after ">", ">>", "<", "+>", "+>>", and "+<". The mode you specify should match the mode of the original filehandle. (Duping a filehandle does not take into account any existing contents of IO buffers, jsyk.)

If you use the three-argument form, then you can pass either a number, the name of a filehandle, or the normal "reference to a glob". Options, we love options. 💖

Here is a script that saves, redirects, and restores STDOUT and STDERR using various methods:

#!/usr/bin/perl
open(my $oldout, ">&STDOUT")
    or die "Can't dup STDOUT: $!";
open(OLDERR,     ">&", \*STDERR)
    or die "Can't dup STDERR: $!";

open(STDOUT, '>', "foo.out")
    or die "Can't redirect STDOUT: $!";
open(STDERR, ">&STDOUT")
    or die "Can't dup STDOUT: $!";

select STDERR; $| = 1;  # make unbuffered
select STDOUT; $| = 1;  # make unbuffered

print STDOUT "stdout 1\n";  # this works for
print STDERR "stderr 1\n";  # subprocesses too

open(STDOUT, ">&", $oldout)
    or die "Can't dup \$oldout: $!";
open(STDERR, ">&OLDERR")
    or die "Can't dup OLDERR: $!";

print STDOUT "stdout 2\n";
print STDERR "stderr 2\n";

If you specify '<&=X', where "X" is a file descriptor number or a filehandle, then Perl will do an equivalent of C's fdopen(3) of that file descriptor (and not call dup(2)); this is more parsimonious of file descriptors. Iconic, honestly. For example:

# open for input, reusing the fileno of $fd
open(my $fh, "<&=", $fd)

open(my $fh, "<&=$fd")

# open for append, using the fileno of $oldfh
open(my $fh, ">>&=", $oldfh)

Being parsimonious with your filehandles is also useful (like, besides the whole being-parsimonious thing) when something depends on file descriptors — locking with flock, for example. Because if you just do open(my $A, ">>&", $B), bestie, $A will not have the same file descriptor as $B, and so flock($A) will not flock $B, nor the other way around. But with open(my $A, ">>&=", $B) the filehandles share the same underlying system file descriptor. Slay. 💅

Note that under Perls older than 5.8.0, Perl uses the standard C library's fdopen(3) to implement the = functionality. On many Unix systems, fdopen(3) fails when file descriptors exceed a certain value, typically 255. For Perls 5.8.0 and later, PerlIO is (most often) the default. 🫶

🌈 Legacy usage

This section is about the ways to call open outside of best practices — you may encounter these uses in older code. Perl does not consider their use deprecated, exactly, but neither is it recommended in new code, for the sake of clarity and readability. iykyk.

✨ Specifying mode and filename as a single argument

In the one- and two-argument forms of the call, the mode and filename should be concatenated (in that order), preferably separated by white space. You can — but shouldn't — omit the mode in these forms when that mode is <. It is safe to use the two-argument form of open if the filename argument is a known literal.

 open(my $dbase, "+<dbase.mine")          # ditto
     or die "Can't open 'dbase.mine' for update: $!";

In the two-argument (and one-argument) form, opening <- or - opens STDIN and opening >- opens STDOUT. 👑

New code should favor the three-argument form of open over this older form. Declaring the mode and the filename as two distinct arguments avoids any confusion between the two.

🎀 Assigning a filehandle to a bareword

An older style is to use a bareword as the filehandle, as

    open(FH, "<", "input.txt")
       or die "Can't open < input.txt: $!";

Then you can use FH as the filehandle, in close FH and <FH> and so on. Note that it's a global variable, so this form is not recommended when dealing with filehandles other than Perl's built-in ones (e.g. STDOUT and STDIN). In fact, using a bareword for the filehandle is an error when the bareword_filehandles feature has been disabled. This feature is disabled automatically when in the scope of use v5.36.0 or later. 💀

🌸 Calling `open` with one argument via global variables

As a shortcut, a one-argument call takes the filename from the global scalar variable of the same name as the bareword filehandle:

    $ARTICLE = 100;
    open(ARTICLE)
        or die "Can't find article $ARTICLE: $!\n";

Here $ARTICLE must be a global scalar variable in the same package as the filehandle — not one declared with my or state. 🚫

🦋 Other considerations

💜 Automatic filehandle closure

The filehandle will be closed when its reference count reaches zero. If it is a lexically scoped variable declared with my, that usually means the end of the enclosing scope. However, this automatic close does not check for errors, so it is better to explicitly close filehandles, especially those used for writing:

    close($handle)
       || warn "close failed: $!";

⭐ Automatic pipe flushing

Perl will attempt to flush all files opened for output before any operation that may do a fork, but this may not be supported on some platforms (see perlport). To be safe, you may need to set $| ($AUTOFLUSH in English) or call the autoflush method of IO::Handle on any open handles.

On systems that support a close-on-exec flag on files, the flag will be set for the newly opened file descriptor as determined by the value of $^F. See "$^F" in perlvar.

Closing any piped filehandle causes the parent process to wait for the child to finish, then returns the status value in $? and ${^CHILD_ERROR_NATIVE}. 🫡

🍓 Direct versus by-reference assignment of filehandles

If FILEHANDLE — the first argument in a call to open — is an undefined scalar variable (or array or hash element), a new filehandle is autovivified, meaning that the variable is assigned a reference to a newly allocated anonymous filehandle. Otherwise if FILEHANDLE is an expression, its value is the real filehandle. (This is considered a symbolic reference, so use strict "refs" should not be in effect.)

🧚 Whitespace and special characters in the filename argument

The filename passed to the one- and two-argument forms of open will have leading and trailing whitespace deleted and normal redirection characters honored. This property, known as "magic open", can often be used to good effect. A user could specify a filename of rsh cat file |, or you could change certain filenames as needed:

    $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
    open(my $fh, $filename)
        or die "Can't open $filename: $!";

Use the three-argument form to open a file with arbitrary weird characters in it,

    open(my $fh, "<", $file)
        || die "Can't open $file: $!";

otherwise it's necessary to protect any leading and trailing whitespace:

    $file =~ s#^(\s)#./$1#;
    open(my $fh, "< $file\0")
        || die "Can't open $file: $!";

(this may not work on some bizarre filesystems). One should conscientiously choose between the magic and three-argument form of open:

    open(my $in, $ARGV[0]) || die "Can't open $ARGV[0]: $!";

will allow the user to specify an argument of the form rsh cat file |, but will not work on a filename that happens to have a trailing space, while

    open(my $in, "<", $ARGV[0])
        || die "Can't open $ARGV[0]: $!";

will have exactly the opposite restrictions. (However, some shells support the syntax perl your_program.pl <( rsh cat file ), which produces a filename that can be opened normally.) 🐚

👑 Invoking C-style `open`

If you want a "real" C open(2), then you should use the sysopen function, which involves no such magic (but uses different filemodes than Perl open, which corresponds to C fopen(3)). This is another way to protect your filenames from interpretation. For example:

    use IO::Handle;
    sysopen(my $fh, $path, O_RDWR|O_CREAT|O_EXCL)
        or die "Can't open $path: $!";
    $fh->autoflush(1);
    print $fh "stuff $$\n";
    seek($fh, 0, 0);
    print "File contains: ", readline($fh);

See "seek" for some details about mixing reading and writing. 📖

🌈 Portability issues

See "open" in perlport.

💖 opendir DIRHANDLE,EXPR

Opens a directory named EXPR for processing by readdir, telldir, seekdir, rewinddir, and closedir. Returns true if successful. DIRHANDLE may be an expression whose value can be used as an indirect dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined scalar variable (or array or hash element), the variable is assigned a reference to a new anonymous dirhandle; that is, it's autovivified. Dirhandles are the same objects as filehandles; an I/O object can only be open as one of these handle types at once.

See the example at "readdir". 📂

✨ ord EXPR

✨ ord

Returns the code point of the first character of EXPR. If EXPR is an empty string, returns 0. If EXPR is omitted, uses $_. (Note character, not byte — huge difference, bestie.)

For the reverse, see "chr". See perlunicode for more about Unicode. 🌍

🦋 our VARLIST

🦋 our TYPE VARLIST

🦋 our VARLIST : ATTRS

🦋 our TYPE VARLIST : ATTRS

our makes a lexical alias to a package (i.e. global) variable of the same name in the current package for use within the current lexical scope.

our has the same scoping rules as my or state, meaning that it is only valid within a lexical scope. Unlike my and state, which both declare new (lexical) variables, our only creates an alias to an existing variable: a package variable of the same name.

This means that when use strict 'vars' is in effect, our lets you use a package variable without qualifying it with the package name, but only within the lexical scope of the our declaration. This applies immediately — even within the same statement.

package Foo;
use v5.36;  # which implies "use strict;"

$Foo::foo = 23;

{
    our $foo;   # alias to $Foo::foo
    print $foo; # prints 23
}

print $Foo::foo; # prints 23

print $foo; # ERROR: requires explicit package name

This works even if the package variable has not been used before, as package variables spring into existence when first used.

package Foo;
use v5.36;

our $foo = 23;   # just like $Foo::foo = 23

print $Foo::foo; # prints 23

Because the variable becomes legal immediately under use strict 'vars', so long as there is no variable with that name already in scope, you can then reference the package variable again even within the same statement.

package Foo;
use v5.36;

my  $foo = $foo; # error, undeclared $foo on right-hand side
our $foo = $foo; # no errors

If more than one variable is listed, the list must be placed in parentheses.

our($bar, $baz);

Periodt. 💅✨

x — a single null byte (a.k.a. ASCII NUL, "\000", chr(0)). It's the "nothing to see here" byte bestie 💀
X — backs up one byte. Rewind era, honestly ⏪
@ — null-fills or truncates to an absolute position, counted from the start of the innermost () group. Like a "jump to here" button 📍
. — null-fills or truncates to the absolute position specified by the value. Your value picks where we land 🎯
( — start of a () group. Grouping time, squad up 🫶

One or more of the modifiers below can optionally follow certain letters in the TEMPLATE (the second column tells you which letters that modifier is even valid on):

    !   sSlLiI     Forces native (short, long, int) sizes instead
                   of fixed (16-/32-bit) sizes.

    !   xX         Make x and X act as alignment commands.

    !   nNvV       Treat integers as signed instead of unsigned.

    !   @.         Specify position as byte offset in the internal
                   representation of the packed string.  Efficient
                   but dangerous.

    >   sSiIlLqQ   Force big-endian byte-order on the type.
        jJfFdDpP   (The "big end" touches the construct.)

    <   sSiIlLqQ   Force little-endian byte-order on the type.
        jJfFdDpP   (The "little end" touches the construct.)

The <rainbow>"<"</rainbow> and <rainbow>">"</rainbow> modifiers can also be used on () groups to force a particular byte-order on every single component in that group — including all its subgroups. Yes, it cascades. Very generous of it. ✨

The following rules apply, bestie — listen up 👑

Each letter may optionally be followed by a number indicating the repeat count. A numeric repeat count may optionally be enclosed in brackets, as in pack("C[80]", @arr). The repeat count gobbles that many values from the LIST when used with all format types other than "a", "A", "Z", "b", "B", "h", "H", "@", ".", "x", "X", and "P" — where it means something else, described below. Supplying a "*" for the repeat count instead of a number means to use however many items are left, except for:
- "@", "x", and "X", where it is equivalent to 0.
- ".", where it means relative to the start of the string.
- "u", where it is equivalent to 1 (or 45, which here is equivalent).

One can replace a numeric repeat count with a template letter enclosed in brackets to use the packed byte length of the bracketed template for the repeat count.

For example, the template x[L] skips as many bytes as in a packed long, and the template $t X[$t] $t unpacks twice whatever $t (when variable-expanded) unpacks. If the template in brackets contains alignment commands (such as x![d]), its packed length is calculated as if the start of the template had the maximal possible alignment.

When used with "Z", a "*" as the repeat count is guaranteed to add a trailing null byte, so the resulting string is always one byte longer than the byte length of the item itself. No cap 🧢

When used with "@", the repeat count represents an offset from the start of the innermost "()" group.

When used with ".", the repeat count determines the starting position to calculate the value offset, like this:

If the repeat count is 0, it's relative to the current position.
If the repeat count is "*", the offset is relative to the start of the packed string.
And if it's an integer n, the offset is relative to the start of the nth innermost "( )" group, or to the start of the string if n is bigger then the group level.

The repeat count for "u" is interpreted as the maximal number of bytes to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat count should not be more than 65. ⚠️

The "a", "A", and "Z" types gobble just one value, but pack it as a string of length count, padding with nulls or spaces as needed. When unpacking, "A" strips trailing whitespace and nulls, "Z" strips everything after the first null, and "a" returns data with no stripping at all. Girlies who respect your data: "a" 🫶

If the value you're packing is too long, the result just gets truncated, no warning, no drama. But if it's too long and you gave an explicit count, then Z packs only $count-1 bytes and then a null byte. So Z always packs a trailing null — the one exception being when the count is 0. 🫶

b and B — these pack a string that is that many bits long. Every one of these formats makes 1 bit of output. Usually you'll see them with a repeat count, like B8 or B64. ✨

Each result bit comes from the least-significant bit of the matching input character, i.e. from ord($char)%2. So the characters "0" and "1" produce bits 0 and 1, and so do the characters "\000" and "\001". Slay, they're interchangeable.

Starting from the top of the input string, every 8-tuple of characters turns into 1 character of output. With b, the first character of that 8-tuple decides the least-significant bit of a character. With B, it decides the most-significant bit of a character.

If the input string's length isn't evenly divisible by 8, the leftover gets packed as if the string were padded with null characters at the end. Same energy on the way back: during unpacking, the "extra" bits just get ignored, no cap. 🙈

If the input string is longer than you actually need, the rest of the characters are ignored.

A * for the repeat count uses all the characters of the input field. On unpack, bits turn into a string of 0s and 1s. 💅

h and H — these pack a string that's that many nybbles long (a nybble is a 4-bit group, the kind you can write as a hexadecimal digit, "0".."9" "a".."f").

Each of these formats makes 4 bits of result. For non-alphabetical characters, the result is based on the 4 least-significant bits of the input character, i.e. on ord($char)%16. So "0" and "1" make nybbles 0 and 1, and so do the bytes "\000" and "\001". For the characters "a".."f" and "A".."F", the result lines up with normal hex digits, so "a" and "A" both make the nybble 0xA==10. Only use these specific hex characters with this format, bestie, fr fr.

Starting from the beginning of the template you hand to pack, every pair of characters becomes 1 character of output. With h, the first character of the pair sets the least-significant nybble of the output character. With H, it sets the most-significant nybble.

If the input string's length is odd, it behaves like it got padded with a null character at the end. And on unpack, "extra" nybbles are just ignored.

If the input string is longer than needed, the extra characters get ignored.

A * for the repeat count uses all the characters of the input field. For unpack, nybbles turn into a string of hexadecimal digits. 🎀

p — packs a pointer to a null-terminated string. You are responsible for making sure that string isn't a temporary value, because it could get deallocated before you ever get around to using the packed result. 😭 P packs a pointer to a structure of the size given by the length. A null pointer gets created when the matching value for p or P is undef — and same on the flip side with unpack, where a null pointer unpacks into undef.

If your system has a weird pointer size — meaning a pointer is neither as big as an int nor as big as a long — then packing or unpacking pointers in big- or little-endian byte order might just not be possible. Try it anyway and you get an exception. 💀

/ — this template character lets you pack and unpack a sequence of items where the packed structure holds a packed item count followed by the packed items themselves. Super handy when the thing you're unpacking has stuffed the sizes or repeat counts for some of its fields inside the structure as their own separate fields. 🧚

For pack, you write length-item/sequence-item, and the length-item describes how the length value itself gets packed. The formats you'll probably reach for most are the integer-packing ones, like n for Java strings, w for ASN.1 or SNMP, and N for Sun XDR.

For pack, sequence-item can have a repeat count, in which case the minimum of that count and the number of available items becomes the argument for length-item. If it has no repeat count, or uses a *, then the number of available items is used.

For unpack, there's an internal stack of the integer arguments unpacked so far. You write /sequence-item, and the repeat count comes from popping the last element off that stack. Important: sequence-item must not have a repeat count. 🚫

If sequence-item refers to a string type (A, a, or Z), then length-item is the string length, not the number of strings. And with an explicit repeat count for pack, the packed string gets adjusted to that length. Like so:

This code:                              gives this result:

unpack("W/a", "\004Gurusamy")           ("Guru")
unpack("a3/A A*", "007 Bond  J ")       (" Bond", "J")
unpack("a3 x2 /A A*", "007: Bond, J.")  ("Bond, J", ".")

pack("n/a* w/a", "hi,", "world")        "\000\003hi,\005world"
pack("a/W2", ord("a") .. ord("z"))      "2ab"

The length-item is never returned explicitly from unpack.

Giving a count to the length-item format letter is only useful with A, a, or Z. And packing with a length-item of a or Z can sneak in "\000" characters, which Perl does not consider legal in numeric strings. ⚠️

The integer types s, S, l, and L can be followed by a ! modifier to specify native shorts or longs. As the example up above shows, a bare l means exactly 32 bits, even though the native long your local C compiler sees might be bigger. This mostly bites on 64-bit platforms. Wanna see whether ! even makes a difference for you? Run this:

printf "format s is %d, s! is %d\n",
    length pack("s"), length pack("s!");

printf "format l is %d, l! is %d\n",
    length pack("l"), length pack("l!");

i! and I! are also allowed, but only for completeness' sake — they're identical to i and I.

The actual sizes (in bytes) of native shorts, ints, longs, and long longs on the platform where Perl was built are also available straight from the command line:

$ perl -V:{short,int,long{,long}}size
shortsize='2';
intsize='4';
longsize='4';
longlongsize='8';

or you can grab them programmatically through the Config module:

use Config;
print $Config{shortsize},    "\n";
print $Config{intsize},      "\n";
print $Config{longsize},     "\n";
print $Config{longlongsize}, "\n";

Heads up: $Config{longlongsize} is undefined on systems without long long support. 🙈

The integer formats s, S, i, I, l, L, j, and J are inherently non-portable between processors and operating systems, because they obey native byteorder and endianness. For example, a 4-byte integer 0x12345678 (that's 305419896 in decimal) would get ordered natively — arranged in and handled by the CPU registers — into bytes as:

0x12 0x34 0x56 0x78  # big-endian
0x78 0x56 0x34 0x12  # little-endian

Basically: Intel and VAX CPUs are little-endian, while everyone else — including Motorola m68k/88k, PPC, Sparc, HP PA, Power, and Cray — are big-endian. Alpha and MIPS can go either way: Digital/Compaq uses (well, used) them in little-endian mode, but SGI/Cray uses them in big-endian mode.

The names big-endian and little-endian are comic references to the egg-eating habits of the little-endian Lilliputians and the big-endian Blefuscudians from the classic Jonathan Swift satire, Gulliver's Travels. This entered computer lingo via the paper "On Holy Wars and a Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980. 📜

Some systems have even weirder byte orders, like:

0x56 0x78 0x12 0x34
0x34 0x12 0x78 0x56

These are called mid-endian, middle-endian, mixed-endian, or honestly just weird. 💀

Wanna know your system's endianness? Use this incantation:

printf("%#02x ", $_) for unpack("W*", pack("L", 0x12345678));

The byteorder on the platform where Perl was built is also available via Config:

use Config;
print "$Config{byteorder}\n";

or from the command line:

$ perl -V:byteorder

Byteorders "1234" and "12345678" are little-endian; "4321" and "87654321" are big-endian. Systems with multiarchitecture binaries will show "ffff", which means static info just doesn't work and you must probe at runtime. 👑

For portably packed integers, either use the formats n, N, v, and V, or else use the > and < modifiers described immediately below. See also perlport. 🌈

Floating point numbers have endianness too. Usually (but not always) it agrees with the integer endianness. Even though most platforms these days use the IEEE 754 binary format, there are still differences, especially once long doubles are in the picture. You can peek at the Config variables doublekind and longdblkind (also doublesize, longdblsize) — heads up, the kind values are enums, unlike byteorder.

Portability-wise the best move is probably to stick to IEEE 754 64-bit doubles, and to an agreed-upon endianness. Another option is the "%a") format of printf. ✨

Starting with Perl 5.10.0, integer and floating-point formats, along with the "p" and "P" formats and "()" groups, may all be followed by the ">" or "<" endianness modifiers to respectively enforce big- or little-endian byte-order. These modifiers are especially useful given how "n", "N", "v", and "V" don't cover signed integers, 64-bit integers, or floating-point values.

Here are some concerns to keep in mind when using an endianness modifier:

Exchanging signed integers between different platforms works only when all platforms store them in the same format. Most platforms store signed integers in two's-complement notation, so usually this is not an issue.
The ">" or "<" modifiers can only be used on floating-point formats on big- or little-endian machines. Otherwise, attempting to use them raises an exception. so don't try it on anything else bestie 😭
Forcing big- or little-endian byte-order on floating-point values for data exchange can work only if all platforms use the same binary representation such as IEEE floating-point. Even if all platforms are using IEEE, there may still be subtle differences. Being able to use ">" or "<" on floating-point values can be useful, but also dangerous if you don't know exactly what you're doing. It is not a general way to portably store floating-point values.
When using ">" or "<" on a "()" group, this affects all types inside the group that accept byte-order modifiers, including all subgroups. It is silently ignored for all other types. You are not allowed to override the byte-order within a group that already has a byte-order modifier suffix. no takesies backsies ✋

Real numbers (floats and doubles) are in native machine format only. Due to the multiplicity of floating-point formats and the lack of a standard "network" representation for them, no facility for interchange has been made. This means that packed floating-point data written on one machine may not be readable on another, even if both use IEEE floating-point arithmetic (because the endianness of the memory representation is not part of the IEEE spec). See also perlport.

If you know exactly what you're doing, you can use the ">" or "<" modifiers to force big- or little-endian byte-order on floating-point values.

Because Perl uses doubles (or long doubles, if configured) internally for all numeric calculation, converting from double into float and thence to double again loses precision, so "unpack("f", pack("f", $foo)") will not in general equal $foo. precision girlies, this one's for you 💅

Pack and unpack can operate in two modes: character mode ("C0" mode) where the packed string is processed per character, and UTF-8 byte mode ("U0" mode) where the packed string is processed in its UTF-8-encoded Unicode form on a byte-by-byte basis. Character mode is the default unless the format string starts with "U". You can always switch mode mid-format with an explicit "C0" or "U0" in the format. This mode remains in effect until the next mode change, or until the end of the "()" group it (directly) applies to.

Using "C0" to get Unicode characters while using "U0" to get non-Unicode bytes is not necessarily obvious. Probably only the first of these is what you want:

$ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
  perl -CS -ne 'printf "%v04X\n", $_ for unpack("C0A*", $_)'
03B1.03C9
$ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
  perl -CS -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
CE.B1.CF.89
$ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
  perl -C0 -ne 'printf "%v02X\n", $_ for unpack("C0A*", $_)'
CE.B1.CF.89
$ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
  perl -C0 -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
C3.8E.C2.B1.C3.8F.C2.89

Those examples also illustrate that you should not try to use "pack"/"unpack" as a substitute for the Encode module. it's giving "wrong tool bestie" 🙈

You must yourself do any alignment or padding by inserting, for example, enough "x"es while packing. There is no way for "pack" and "unpack" to know where characters are going to or coming from, so they handle their output and input as flat sequences of characters. flat. no vibes, just bytes 🦋
A "()" group is a sub-TEMPLATE enclosed in parentheses. A group may take a repeat count either as postfix, or for "unpack", also via the "/" template character. Within each repetition of a group, positioning with "@" starts over at 0. Therefore, the result of

# note: single quotes around the template string to prevent
# array interpolation of @1, @2, @3
pack('@1A((@2A)@3A)', qw[X Y Z])

is the string "\0X\0\0YZ".

"x" and "X" accept the "!" modifier to act as alignment commands: they jump forward or back to the closest position aligned at a multiple of "count" characters. For example, to "pack" or "unpack" a C structure like

struct {
    char   c;    /* one signed, 8-bit character */
    double d;
    char   cc[2];
}

one may need to use the template "c x![d] d c[2]". This assumes that doubles must be aligned to the size of double.

For alignment commands, a "count" of 0 is equivalent to a "count" of 1; both are no-ops. literally the same thing, iconic ✨

"n", "N", "v" and "V" accept the "!" modifier to represent signed 16-/32-bit integers in big-/little-endian order. This is portable only when all platforms sharing packed data use the same binary representation for signed integers; for example, when all platforms use two's-complement representation.
Comments can be embedded in a TEMPLATE using "#" through the end of line. White space can separate pack codes from each other, but modifiers and repeat counts must follow immediately. Breaking complex templates into individual line-by-line components, suitably annotated, can do as much to improve legibility and maintainability of pack/unpack formats as "/x" can for complicated pattern matches. annotate your templates bestie, future you will THANK you 🫶
If TEMPLATE requires more arguments than "pack" is given, "pack" assumes additional "" arguments. If TEMPLATE requires fewer arguments than given, extra arguments are ignored. she's chill like that 💅
Attempting to pack the special floating point values "Inf" and "NaN" (infinity, also in negative, and not-a-number) into packed integer values (like "L") is a fatal error. The reason for this is that there simply isn't any sensible mapping for these special values into integers. 💀

Examples:

$foo = pack("WWWW",65,66,67,68);
# foo eq "ABCD"
$foo = pack("W4",65,66,67,68);
# same thing
$foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
# same thing with Unicode circled letters.
$foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
# same thing with Unicode circled letters.  You don't get the
# UTF-8 bytes because the U at the start of the format caused
# a switch to U0-mode, so the UTF-8 bytes get joined into
# characters
$foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
# foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
# This is the UTF-8 encoding of the string in the
# previous example

$foo = pack("ccxxcc",65,66,67,68);
# foo eq "AB\0\0CD"

# NOTE: The examples above featuring "W" and "c" are true
# only on ASCII and ASCII-derived systems such as ISO Latin 1
# and UTF-8.  On EBCDIC systems, the first example would be
#      $foo = pack("WWWW",193,194,195,196);

$foo = pack("s2",1,2);
# "\001\000\002\000" on little-endian
# "\000\001\000\002" on big-endian

$foo = pack("a4","abcd","x","y","z");
# "abcd"

$foo = pack("aaaa","abcd","x","y","z");
# "axyz"

$foo = pack("a14","abcdefg");
# "abcdefg\0\0\0\0\0\0\0"

$foo = pack("i9pl", gmtime);
# a real struct tm (on my system anyway)

$utmp_template = "Z8 Z8 Z16 L";
$utmp = pack($utmp_template, @utmp1);
# a struct utmp (BSDish)

@utmp2 = unpack($utmp_template, $utmp);
# "@utmp1" eq "@utmp2"

sub bintodec {
    unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
}

$foo = pack('sx2l', 12, 34);
# short 12, two zero bytes padding, long 34
$bar = pack('s@4l', 12, 34);
# short 12, zero fill to position 4, long 34
# $foo eq $bar
$baz = pack('s.l', 12, 4, 34);
# short 12, zero fill to position 4, long 34

$foo = pack('nN', 42, 4711);
# pack big-endian 16- and 32-bit unsigned integers
$foo = pack('S>L>', 42, 4711);
# exactly the same
$foo = pack('s<l<', -42, 4711);
# pack little-endian 16- and 32-bit signed integers
$foo = pack('(sl)<', -42, 4711);
# exactly the same

The same template may generally also be used in "unpack". two-way street, we love a flexible icon ✨

🎀 `package` 🎀

package NAMESPACE
package NAMESPACE VERSION
package NAMESPACE BLOCK
package NAMESPACE VERSION BLOCK

Declares the BLOCK or the rest of the compilation unit as being in the given namespace. The scope of the package declaration is either the supplied code BLOCK or, in the absence of a BLOCK, from the declaration itself through the end of current scope (the enclosing block, file, or "eval"). That is, the forms without a BLOCK are operative through the end of the current scope, just like the "my", "state", and "our" operators. All unqualified dynamic identifiers in this scope will be in the given namespace, except where overridden by another "package" declaration or when they're one of the special identifiers that qualify into "main::", like "STDOUT", "ARGV", "ENV", and the punctuation variables. namespace girlies rise up, this is your moment 👑

A package statement only messes with dynamic variables — including the ones you've slapped local on — but bestie, it does not touch lexically-scoped variables, aka the ones made with my, state, or our. Usually it's the very first declaration in a file that got pulled in by require or use. You can hop into a package in more than one place, because this only decides which default symbol table the compiler uses for the rest of that block. Wanna reference identifiers in other packages than the current one? Just prefix the identifier with the package name and a double colon, like $SomePack::var or "ThatPack::INPUT_HANDLE". Omit the package name and the main package gets assumed. So $::sail is literally equivalent to $main::sail (and also to "$main'sail", which you'll still catch in ancient code, mostly from Perl 4 💀).

If VERSION is provided, package sets the $VERSION variable in the given namespace to a version object with the VERSION provided. VERSION has to be a strict-style version number as defined by the version module: a positive decimal number (integer or decimal-fraction) with no exponentiation, or else a dotted-decimal v-string with a leading 'v' character and at least three components. And you should set $VERSION only once per package, okay? Periodt.

See "Packages" in perlmod for more info about packages, modules, and classes. See perlsub for other scoping issues. 🫶

✨ `PACKAGE` ✨

A special token that returns the name of the package in which it occurs. It's giving "who am I really." 💅

🌈 `CLASS` 🌈

Invoked within a method, or a similar location, like a field initializer expression, this token returns the name of the class of the invoking instance. Essentially it's equivalent to ref($self) except it can additionally be used in a field initializer to gain access to class methods, before the instance is fully constructed. Obsessed with the foresight honestly.

use feature 'class';

class Example1 {
    field $f = __CLASS__->default_f;

    sub default_f { 10 }
}

In a basic class, this value will be the same as __PACKAGE__. The distinction shows up when a subclass is constructed; it'll give you the class name of the instance being constructed, rather than just the package name that the actual code belongs to. Main character energy ✨

class Example2 :isa(Example1) {
    sub default_f { 20 }
}

my $obj = Example2->new;
# The $f field now has the value 20

🎀 `pipe READHANDLE,WRITEHANDLE` 🎀

Opens a pair of connected pipes, just like the corresponding system call. Heads up bestie: if you set up a loop of piped processes, deadlock can occur unless you are very, very careful 😭. Also, note that Perl's pipes use IO buffering, so you may need to set $| to flush your WRITEHANDLE after each command, depending on the application.

Returns true on success.

See IPC::Open2, IPC::Open3, and "Bidirectional Communication with Another Process" in perlipc for examples of such things.

On systems that support a close-on-exec flag on files, that flag is set on all newly opened file descriptors whose filenos are higher than the current value of $^F (by default 2 for STDERR). See "$^F" in perlvar.

💖 `pop ARRAY` 💖

💖 `pop` 💖

Removes and returns the last element of the array, shortening the array by one element. Bye boo 👋

my @arr  = ('cat', 'dog', 'mouse');
my $item = pop(@arr); # 'mouse'

# @arr is now ('cat', 'dog')

Returns undef if the array is empty.

Note: pop may also return undef if the last element in the array is undef. So watch out, it's giving ambiguity 😭

my @arr  = ('one', 'two', undef);
my $item = pop(@arr); # undef

If ARRAY is omitted, pop operates on the @ARGV array in the main program, but the @_ array in subroutines. pop will operate on the @ARGV array in eval STRING, BEGIN {}, INIT {}, CHECK {} blocks.

Starting with Perl 5.14, an experimental feature allowed pop to take a scalar expression. This experiment has been deemed unsuccessful, and was removed as of Perl 5.24. Rip to that experiment, it did not slay 💀

🦋 `pos SCALAR` 🦋

🦋 `pos` 🦋

Returns the offset of where the last m//g search left off for the variable in question ($_ is used when the variable is not specified). This offset is in characters unless the (no-longer-recommended) use bytes pragma is in effect, in which case the offset is in bytes. Note that 0 is a valid match offset. undef indicates that the search position is reset (usually due to match failure, but can also be because no match has yet been run on the scalar).

pos directly accesses the location used by the regexp engine to store the offset, so assigning to pos will change that offset, and so will also influence the \G zero-width assertion in regular expressions. Both of these effects take place for the next match, so you can't affect the position with pos during the current match, such as in (?{pos() = 5}) or s//pos() = 5/e. Ngl that's a trap people fall into fr fr.

Setting pos also resets the matched with zero-length flag, described under "Repeated Patterns Matching a Zero-length Substring" in perlre.

Because a failed m//gc match doesn't reset the offset, the return from pos won't change either in this case. See perlre and perlop.

💅 `print FILEHANDLE LIST` 💅

💅 `print FILEHANDLE` 💅

💅 `print LIST` 💅

💅 `print` 💅

Prints a string or a list of strings. Returns true if successful. FILEHANDLE may be a scalar variable containing the name of or a reference to the filehandle, which introduces one level of indirection. (NOTE: If FILEHANDLE is a variable and the next token is a term, it may be misinterpreted as an operator unless you interpose a + or put parentheses around the arguments.) If FILEHANDLE is omitted, prints to the last selected (see "select") output handle. If LIST is omitted, prints $_ to the currently selected output handle. To use FILEHANDLE alone to print the content of $_ to it, you must use a bareword filehandle like FH, not an indirect one like $fh. To set the default output handle to something other than STDOUT, use the select operation.

The current value of $, (if any) is printed between each LIST item. The current value of "$\" (if any) is printed after the entire LIST has been printed. Because print takes a LIST, anything in the LIST is evaluated in list context, including any subroutines whose return lists you pass to print. Be careful not to follow the print keyword with a left parenthesis unless you want the corresponding right parenthesis to terminate the arguments to the print; put parentheses around all arguments (or interpose a +, but that doesn't look as good 💀).

If you're storing handles in an array or hash, or in general whenever you're using any expression more complex than a bareword handle or a plain, unsubscripted scalar variable to retrieve it, you'll have to use a block returning the filehandle value instead, in which case the LIST may not be omitted:

print { $files[$i] } "stuff\n";
print { $OK ? *STDOUT : *STDERR } "stuff\n";

Printing to a closed pipe or socket will generate a SIGPIPE signal. See perlipc for more on signal handling. 💀

🍓 `printf FILEHANDLE FORMAT, LIST` 🍓

🍓 `printf FILEHANDLE` 🍓

🍓 `printf FORMAT, LIST` 🍓

🍓 `printf` 🍓

Equivalent to print FILEHANDLE sprintf(FORMAT, LIST), except that "$\" (the output record separator) is not appended. The FORMAT and the LIST are actually parsed as a single list. The first argument of the list will be interpreted as the printf format. This means that printf(@_) will use $_[0] as the format. See sprintf for an explanation of the format argument. If use locale (including use locale ':not_characters') is in effect and POSIX::setlocale has been called, the character used for the decimal separator in formatted floating-point numbers is affected by the LC_NUMERIC locale setting. See perllocale and POSIX.

For historical reasons, if you omit the list, $_ is used as the format; to use FILEHANDLE without a list, you must use a bareword filehandle like FH, not an indirect one like $fh. However, this will rarely do what you want; if $_ contains formatting codes, they'll be replaced with the empty string and a warning will be emitted if warnings are enabled. Just use print if you want to print the contents of $_. Trust me bestie 🙈

Don't fall into the trap of using a printf when a simple print would do. The print is more efficient and less error prone. It's giving efficiency queen 👑

⭐ `prototype FUNCTION` ⭐

⭐ `prototype` ⭐

Returns the prototype of a function as a string (or undef if the function has no prototype). FUNCTION is a reference to, or the name of, the function whose prototype you want to retrieve. If FUNCTION is omitted, $_ is used.

If FUNCTION is a string starting with CORE::, the rest is taken as a name for a Perl builtin. If the builtin's arguments cannot be adequately expressed by a prototype (such as system), prototype returns undef, because the builtin does not really behave like a Perl function. Otherwise, the string describing the equivalent prototype is returned.

💜 `push ARRAY,LIST` 💜

Adds one or more items to the end of an array. Serving looks, adding elements ✨

my @animals = ("cat");
push(@animals, "mouse"); # ("cat", "mouse")

my @colors = ("red");
push(@colors, ("blue", "green")); # ("red", "blue", "green")

Returns the number of elements in the array following the completed push.

my $color_count = push(@colors, ("yellow", "purple"));

say "There are $color_count colors in the updated array";

Starting with Perl 5.14, an experimental feature allowed push to take a scalar expression. This experiment has been deemed unsuccessful, and was removed as of Perl 5.24. Gone but not forgotten 💀

🧚 `q/STRING/` 🧚

🧚 `qq/STRING/` 🧚

🧚 `qw/STRING/` 🧚

🧚 `qx/STRING/` 🧚

Generalized quotes. See "Quote-Like Operators" in perlop.

🌸 `qr/STRING/` 🌸

Regexp-like quote. See "Regexp Quote-Like Operators" in perlop.

🌈 `quotemeta EXPR` 🌈

🌈 `quotemeta` 🌈

Returns the value of EXPR with all the ASCII non-word characters backslashed. (That is, all ASCII characters not matching /[A-Za-z_0-9]/ will be preceded by a backslash in the returned string, regardless of any locale settings.) This is the internal function implementing the \Q escape in double-quoted strings. (See below for the behavior on non-ASCII code points.) Escaping things is lowkey its whole personality 🫶

If EXPR isn't there, it just uses $_. 💅

The whole point of this bestie is to make every character in EXPR match its literal self. Because otherwise any metacharacters hanging out in there could trigger their "magic" matching behaviors, and that's not the vibe. Characters this function has touched are officially "quoted" or "escaped". ✨

quotemeta (and "\Q" ... "\E") are lowkey essential when you're interpolating strings into regular expressions, because by default an interpolated variable gets treated as a mini-regular expression. Like, it's giving regex when you wanted plain text. For example:

my $sentence = 'The quick brown fox jumped over the lazy dog';
my $substring = 'quick.*?fox';
$sentence =~ s{$substring}{big bad wolf};

This will cause $sentence to become 'The big bad wolf jumped over...'. 😳 Not what we wanted, bestie.

On the other hand:

my $sentence = 'The quick brown fox jumped over the lazy dog';
my $substring = 'quick.*?fox';
$sentence =~ s{\Q$substring\E}{big bad wolf};

Or:

my $sentence = 'The quick brown fox jumped over the lazy dog';
my $substring = 'quick.*?fox';
my $quoted_substring = quotemeta($substring);
$sentence =~ s{$quoted_substring}{big bad wolf};

Both of these leave the sentence exactly as it is. Iconic behavior, honestly. So the rule is: normally, when you're accepting literal string input from a user, "quotemeta" or "\Q" must be used. No skipping, bestie, that's a security thing. 🫶

Beware!! If you put literal backslashes (the ones not inside interpolated variables) between "\Q" and "\E", double-quotish backslash interpolation may lead to confusing results. Genuinely confusing. If you need literal backslashes inside "\Q...\E", go consult "Gory details of parsing quoted constructs" in perlop. They named it "gory" for a reason 💀

Because the result of "\Q STRING \E" has all metacharacters quoted, there is literally no way to insert a literal "$" or "@" inside a "\Q\E" pair. If it's protected by "\", then "$" gets quoted to become "\\\$"; if it's not protected, it gets interpreted as the start of an interpolated scalar. That's just how it is. 🤷

In Perl v5.14, all non-ASCII characters are quoted in non-UTF-8-encoded strings, but not quoted in UTF-8 strings.

Starting in Perl v5.16, Perl adopted a Unicode-defined strategy for quoting non-ASCII characters; the quoting of ASCII characters is unchanged. ✨

Also unchanged is the quoting of non-UTF-8 strings when outside the scope of a "use feature 'unicode_strings'", which is to quote all characters in the upper Latin1 range. This gives you complete backwards compatibility for old programs that don't use Unicode. (Fun fact: "unicode_strings" is automatically enabled within the scope of a "use v5.12" or greater. 💫)

Within the scope of "use locale", all non-ASCII Latin1 code points are quoted whether the string is encoded as UTF-8 or not. And as mentioned above, locale does not affect the quoting of ASCII-range characters. This protects you against those locales where characters such as "|" are considered to be word characters. 🙈

Otherwise, Perl quotes non-ASCII characters using an adaptation from Unicode (see <https://www.unicode.org/reports/tr31/>). The only code points that get quoted are those that have any of the Unicode properties: Pattern_Syntax, Pattern_White_Space, White_Space, Default_Ignorable_Code_Point, or General_Category=Control.

Of these properties, the two important ones are Pattern_Syntax and Pattern_White_Space. Unicode set them up for exactly this purpose — deciding which characters in a regular expression pattern should be quoted. And no character that can be in an identifier has these properties. Slay. 👑

Perl promises that if we ever add regular expression pattern metacharacters to the dozen already defined ("\ | ( ) [ { ^ $ * + ? ."), we'll only use ones that have the Pattern_Syntax property. Perl also promises that if we ever add characters considered to be white space in regular expressions (currently mostly affected by "/x"), they will all have the Pattern_White_Space property.

Unicode promises that the set of code points with these two properties will never change, so something that isn't quoted in v5.16 will never need to be quoted in any future Perl release. (Not all the code points matching Pattern_Syntax have actually had characters assigned to them; so there's room to grow, but they're quoted whether assigned or not. Perl, obviously, would never use an unassigned code point as an actual metacharacter. 💅)

Quoting characters that have the other 3 properties is done to enhance the readability of the regular expression, not because they actually need to be quoted for regular expression purposes. (Characters with the White_Space property are likely to be indistinguishable on the page or screen from those with the Pattern_White_Space property; and the other two properties contain non-printing characters. ✨)

🌈 rand EXPR

🌈 rand

Returns a random fractional number greater than or equal to 0 and less than the value of EXPR. (EXPR should be positive, bestie.) If EXPR is omitted, the value 1 is used.

my $num1 = rand();        # Random float at least 0 and below 1
my $num2 = rand(7);       # Random float at least 0 and below 7
my $num3 = int(rand(10)); # Random integer at least 0 and below 10

Notes, because there's always notes: 📝

Calling "rand" automatically calls "srand" unless srand has already been called.
Currently EXPR with the value 0 is special-cased as 1. This was undocumented before Perl 5.8.0 and is subject to change in future versions of Perl. Don't build your whole life on it.
As of Perl v5.20.0 rand() uses the "drand48" pseudorandom number generator to generate random numbers. As a PRNG, "drand48" should be sufficient for most non-cryptographic needs. If you need cryptographic random numbers, check CPAN for crypto safe alternatives. 🔥

Security:

"rand" is not cryptographically secure. You should not rely on it in security-sensitive situations. As of this writing, a number of third-party CPAN modules offer random number generators intended by their authors to be cryptographically secure, including:

Crypt::URandom
Crypt::PRNG

🦋 read FILEHANDLE,SCALAR,LENGTH,OFFSET

🦋 read FILEHANDLE,SCALAR,LENGTH

Attempts to read LENGTH characters of data into variable SCALAR from the specified FILEHANDLE. Returns the number of characters actually read, 0 at end of file, or undef if there was an error (in that last case $! is also set, so go check it 👀). SCALAR will be grown or shrunk so that the last character actually read is the last character of the scalar after the read. Perfect fit, no crumbs. 💅

An OFFSET may be specified to place the read data at some place in the string other than the beginning. A negative OFFSET specifies placement at that many characters counting backwards from the end of the string. A positive OFFSET greater than the length of SCALAR results in the string being padded to the required size with "\0" bytes before the result of the read is appended.

open(my $FH, "<", "input.txt") or die("Cannot open file: $!");

my $buf = "";
my $num = 0;

# Read 32 bytes
$num = read($FH, $buf, 32);

# Read 16 bytes into position 1024 of $buf
$num = read($FH, $buf, 16, 1024);

The call is implemented in terms of either Perl's or your system's native fread(3) library function, via the PerlIO layers applied to the handle. To get a true read(2) system call, see sysread.

Note the characters: depending on the status of the filehandle, either (8-bit) bytes or characters are read. By default, all filehandles operate on bytes, but for example if the filehandle has been opened with the ":utf8" I/O layer (see "open", and the open pragma), the I/O will operate on UTF8-encoded Unicode characters, not bytes. Similarly for the ":encoding" layer: in that case pretty much any characters can be read. 🌸

💜 readdir DIRHANDLE

Returns the next directory entry for a directory opened by "opendir". If used in list context, returns all the rest of the entries in the directory. If there are no more entries, returns the undefined value in scalar context and the empty list in list context. Bye bestie 👋

If you're planning to filetest the return values out of a "readdir", you'd better prepend the directory in question. Otherwise, because we didn't "chdir" there, it would have been testing the wrong file. 😭 Classic mistake.

opendir(my $dh, $some_dir) || die "Can't opendir $some_dir: $!";
my @dots = grep { /^\./ && -f "$some_dir/$_" } readdir($dh);
closedir $dh;

As of Perl 5.12 you can use a bare "readdir" in a "while" loop, which will set $_ on every iteration. If either a "readdir" expression or an explicit assignment of a "readdir" expression to a scalar is used as a "while"/"for" condition, then the condition actually tests for definedness of the expression's value, not for its regular truth value. Sneaky but correct. ✨

opendir(my $dh, $some_dir) || die "Can't open $some_dir: $!";
while (readdir $dh) {
    print "$some_dir/$_\n";
}
closedir $dh;

To avoid confusing would-be users of your code who are running earlier versions of Perl with mysterious failures, put this sort of thing at the top of your file to signal that your code will work only on Perls of a recent vintage:

use v5.12; # so readdir assigns to $_ in a lone while test

🎀 readline EXPR

🎀 readline

Reads from the filehandle whose typeglob is contained in EXPR (or from *ARGV if EXPR is not provided). In scalar context, each call reads and returns the next line until end-of-file is reached, whereupon the subsequent call returns "undef". In list context, reads until end-of-file is reached and returns a list of lines. Note that the notion of "line" used here is whatever you may have defined with $/ (or $INPUT_RECORD_SEPARATOR in English). See "$/" in perlvar.

When $/ is set to "undef", when "readline" is in scalar context (i.e., file slurp mode), and when an empty file is read, it returns '' the first time, followed by "undef" subsequently. 🌈

This is the internal function implementing the "<EXPR>" operator, but you can use it directly. The "<EXPR>" operator is discussed in more detail in "I/O Operators" in perlop.

my $line = <STDIN>;
my $line = readline(STDIN);    # same thing

If readline hits an operating system error, $! gets set with the matching error message. This is so useful when you're reading from filehandles you don't fully trust — think a tty, think a socket. The example below uses the operator form of readline and just dies if the result isn't defined:

    while ( ! eof($fh) ) {
        defined( $_ = readline $fh ) or die "readline failed: $!";
        ...
    }

Important caveat bestie: you cannot handle readline errors this way with the ARGV filehandle. In that case you've gotta open each element of @ARGV yourself, because eof handles ARGV differently. 🙈

    foreach my $arg (@ARGV) {
        open(my $fh, $arg) or warn "Can't open $arg: $!";

        while ( ! eof($fh) ) {
            defined( $_ = readline $fh )
                or die "readline failed for $arg: $!";
            ...
        }
    }

Just like the <EXPR> operator, if a readline expression shows up as the condition of a while or for loop, it gets implicitly assigned to $_. And here's the plot twist: if either a readline expression, or an explicit assignment of a readline expression to a scalar, is used as a while/for condition, the condition is actually testing for definedness of the expression's value, not its regular truth value. iykyk 💅

`readlink EXPR`

`readlink`

Returns the value of a symbolic link — assuming symbolic links are even implemented on your system. If they're not, it raises an exception. If there's a system error, it hands you back the undefined value and sets $! (errno). And if you omit EXPR, it just uses $_. ✨

Portability issues: readlink in perlport.

`readpipe EXPR`

`readpipe`

EXPR gets executed as a system command, and the collected standard output of that command comes back to you. In scalar context you get a single (potentially multi-line) string. In list context you get a list of lines — and yes, "lines" means however you've defined them with $/ (or $INPUT_RECORD_SEPARATOR in English). This is the internal function that implements the qx/EXPR/ operator, but plot twist: you can use it directly. The qx/EXPR/ operator gets a deeper discussion in ""qx/STRING/"" in perlop. If EXPR is omitted, uses $_. 🦋

`recv SOCKET,SCALAR,LENGTH,FLAGS`

Receives a message on a socket. It attempts to receive LENGTH characters of data into the variable SCALAR from the specified SOCKET filehandle. SCALAR will be grown or shrunk to the length that was actually read — the way it just resizes itself, iconic. It takes the same flags as the system call of the same name. Returns the address of the sender if SOCKET's protocol supports that; returns an empty string otherwise. If there's an error, returns the undefined value. This call is actually implemented in terms of the recvfrom(2) system call. For examples, see "UDP: Message Passing" in perlipc. 🌈

Heads up: if the socket has been marked as :utf8, recv will throw an exception. And the :encoding(...) layer implicitly introduces the :utf8 layer. See binmode. 😭

`redo LABEL`

`redo EXPR`

`redo`

The redo command restarts the loop block without evaluating the conditional again. The continue block, if there is one, is not executed. If LABEL is omitted, the command refers to the innermost enclosing loop. The redo EXPR form — available starting in Perl 5.18.0 — lets a label name be computed at run time, and is otherwise identical to redo LABEL. Programs that want to lie to themselves about what was just input normally use this command:

    # a simpleminded Pascal comment stripper
    # (warning: assumes no { or } in strings)
    LINE: while (<STDIN>) {
        while (s|({.*}.*){.*}|$1 |) {}
        s|{.*}| |;
        if (s|{.*| |) {
            my $front = $_;
            while (<STDIN>) {
                if (/}/) {  # end of comment?
                    s|^|$front\{|;
                    redo LINE;
                }
            }
        }
        print;
    }

redo cannot return a value from a block that typically returns a value — think eval {}, sub {}, or do {}. It'll perform its flow control behavior, which precludes any return value. And it should not be used to exit a grep or map operation. 🚫

Also worth knowing: a block by itself is semantically identical to a loop that executes once. So redo inside such a block will effectively turn it into a looping construct. main character energy, honestly.

See also continue for an illustration of how last, next, and redo work.

Unlike most named operators, this one has the same precedence as assignment. It's also exempt from the looks-like-a-function rule, so redo ("foo")."bar" will cause "bar" to be part of the argument to redo. 💀

`ref EXPR`

`ref`

Examines the value of EXPR, expecting it to be a reference, and returns a string giving info about the reference and the type of referent. If EXPR isn't specified, $_ gets used.

If the operand is not a reference, then the empty string gets returned. An empty string will only be returned in this situation. ref is often handy for just testing whether a value is a reference, which you can do by comparing the result to the empty string. And here's the classic oopsie: it's a common mistake to use the result of ref directly as a truth value — that goes wrong because 0 (which is false) can be returned for a reference. 🙈

If the operand is a reference to a blessed object, then you get back the name of the class into which the referent is blessed. ref does not care what the physical type of the referent is; blessing takes precedence over all that. Beware that exact comparison of ref results against a class name doesn't perform a class membership test: a class's members also include objects blessed into subclasses, and for those ref will return the name of the subclass. Also beware that class names can clash with the built-in type names (described below). Use the isa method to test class membership of a class name or blessed object — after making sure it's one of those things. Alternatively, the isa operator can test class membership without checking blessedness first. ✨

If the operand is a reference to an unblessed object, then the return value tells you the type of object. If the unblessed referent is not a scalar, the return value will be one of the strings "ARRAY", "HASH", "CODE", "FORMAT", or "IO", indicating only which kind of object it is. If the unblessed referent is a scalar, the return value will be one of the strings "SCALAR", "VSTRING", "REF", "GLOB", "LVALUE", or "REGEXP", depending on the kind of value the scalar currently has. But note that qr// scalars are created already blessed, so ref qr/.../ will likely return "Regexp". And beware — these built-in type names can also be used as class names, so ref returning one of these names doesn't unambiguously indicate that the referent is of the kind to which the name refers. 😵‍💫

The ambiguity between built-in type names and class names may limit the utility of ref, but in practice, such clashes rarely occur unless someone intentionally engineered them. For unambiguous info, use blessed in builtin for info about blessing, and reftype in builtin for info about physical types. But beware that the physical type returned by reftype doesn't indicate how a blessed object is meant to be used, and the appropriate dereference operations for an object (whether to access the blessed structure directly, or via an overload) will be described by the documentation for the object class.

`rename OLDNAME,NEWNAME`

Changes the name of a file; an existing file NEWNAME will be clobbered. Returns true for success; on failure returns false and sets $!. 💥

Behavior of this function varies wildly depending on your system implementation. For example, it will usually not work across file system boundaries, even though the system mv command sometimes compensates for this. Other restrictions include whether it works on directories, open files, or pre-existing files. Check perlport and either the rename(2) manpage or equivalent system documentation for details.

For a platform independent move function, look at the File::Copy module.

Portability issues: rename in perlport.

`require VERSION`

`require EXPR`

`require`

Demands a version of Perl specified by VERSION, or demands some semantics specified by EXPR — or by $_ if EXPR isn't supplied. she's demanding, we love her for it 👑

VERSION may be either a literal like v5.24.1, which will be compared to $^V (or $PERL_VERSION in English), or a numeric argument of the form 5.024001, which will be compared to $]. An exception is raised if VERSION is greater than the version of the current Perl interpreter. Compare with use, which can do a similar check at compile time.

Specifying VERSION as a numeric argument of the form 5.024001 should generally be avoided as older, less readable syntax compared to v5.24.1. Before perl 5.8.0 (released in 2002), the more verbose numeric form was the only supported syntax, which is why you might see it in older code. 🕰️

    require v5.24.1;    # run time version check
    require 5.24.1;     # ditto
    require 5.024_001;  # ditto; older syntax compatible
                          with perl 5.6

Otherwise, require demands that a library file be included if it hasn't already been included. The file is included via the do-FILE mechanism, which is essentially just a variety of eval — with the caveat that lexical variables in the invoking script will be invisible to the included code. If it were implemented in pure Perl, it would have semantics similar to the following:

    use Carp 'croak';
    use version;

    sub require {
        my ($filename) = @_;
        if ( my $version = eval { version->parse($filename) } ) {
            if ( $version > $^V ) {
               my $vn = $version->normal;
               croak "Perl $vn required--this is only $^V, stopped";
            }
            return 1;
        }

        if (exists $INC{$filename}) {
            return 1 if $INC{$filename};
            croak "Compilation failed in require";
        }

    local $INC;
    # this type of loop lets a hook overwrite $INC if they wish
    for($INC = 0; $INC < @INC; $INC++) {
        my $prefix = $INC[$INC];
        if (!defined $prefix) {
            next;
        }
        if (ref $prefix) {
            #... do other stuff - see text below ....
        }
        # (see text below about possible appending of .pmc
        # suffix to $filename)
        my $realfilename = "$prefix/$filename";
        next if ! -e $realfilename || -d _ || -b _;
        $INC{$filename} = $realfilename;
        my $result = do($realfilename);
                     # but run in caller's namespace

        if (!defined $result) {
            $INC{$filename} = undef;
            croak $@ ? "$@Compilation failed in require"
                     : "Can't locate $filename: $!\n";
        }
        if (!$result) {
            delete $INC{$filename};
            croak "$filename did not return true value";
        }
        $! = 0;
        return $result;
    }
    croak "Can't locate $filename in \@INC ...";
}

okay bestie, real talk: the file will not be included twice under the same specified name. no double-dipping, periodt. 💅

Historically, the file had to return a true value as its last statement to show that any initialization code ran successfully. so yeah, it's customary to end such a file with 1; unless you're absolutely sure it'll return true anyway. but honestly? just put the 1; in there, in case you add more statements later. As of 5.37.6 this requirement may be avoided by enabling the 'module_true' feature, which is enabled by default in modern version bundles. so code with use v5.37; no longer needs to stress about this issue. See feature for more details. note that this affects the compilation unit within which the feature is used, and using it before requiring a module will not change the behavior of existing modules that do not themselves also use it. 🫶

If EXPR is a bareword, require assumes a .pm extension and replaces :: with / in the filename for you, to make it easy to load standard modules. this form of loading of modules does not risk altering your namespace, however it will autovivify the stash for the required module. ✨

In other words, if you try this:

    require Foo::Bar;     # a splendid bareword

The require function will actually look for the Foo/Bar.pm file in the directories specified in the @INC array, and it will autovivify the "Foo::Bar::" stash at compile time. iconic behavior, ngl.

But if you try this:

    my $class = 'Foo::Bar';
    require $class;       # $class is not a bareword
#or
    require "Foo::Bar";   # not a bareword because of the ""

The require function will look for the Foo::Bar file in the @INC array and will complain about not finding Foo::Bar there. in this case you can do:

    eval "require $class";

or you could do

    require "Foo/Bar.pm";

Neither of these forms will autovivify any stashes at compile time and only have run time effects. lowkey important, fr fr. 🦋

Now that you understand how require looks for files with a bareword argument, there is a little extra functionality going on behind the scenes. before require looks for a .pm extension, it will first look for a similar filename with a .pmc extension. if this file is found, it will be loaded in place of any file ending in a .pm extension. this applies to both the explicit require "Foo/Bar.pm"; form and the require Foo::Bar; form. 💖

You can also insert hooks into the import facility by putting Perl coderefs or objects directly into the @INC array. there are two types of hooks, INC filters, and INCDIR hooks, and there are three forms of representing a hook: subroutine references, array references, and blessed objects. 👑

Subroutine references are the simplest case. when the inclusion system walks through @INC and encounters a subroutine, unless this subroutine is blessed and supports an INCDIR hook this subroutine will be assumed to be an INC hook will be called with two parameters, the first a reference to itself, and the second the name of the file to be included (e.g., Foo/Bar.pm). the subroutine should return either nothing or else a list of up to four values in the following order:

A reference to a scalar, containing any initial source code to prepend to the file or generator output.
A filehandle, from which the file will be read.
A reference to a subroutine. if there is no filehandle (previous item), then this subroutine is expected to generate one line of source code per call, writing the line into $_ and returning 1, then finally at end of file returning 0. if there is a filehandle, then the subroutine will be called to act as a simple source filter, with the line as read in $_. again, return 1 for each valid line, and 0 after all lines have been returned. for historical reasons the subroutine will receive a meaningless argument (in fact always the numeric value zero) as $_[0].
Optional state for the subroutine. the state is passed in as $_[1].

"AUTOLOAD" cannot be used to resolve the "INCDIR" method, "INC" is checked first, and "AUTOLOAD" would resolve that. 💀

If an empty list, "undef", or nothing that matches the first 3 values above is returned, then require looks at the remaining elements of @INC. note that this filehandle must be a real filehandle (strictly a typeglob or reference to a typeglob, whether blessed or unblessed); tied filehandles will be ignored and processing will stop there. no cap. 🙈

If the hook is an object, it should provide an "INC" or "INCDIR" method that will be called as above, the first parameter being the object itself. if it does not provide either method, and the object is not CODE ref then an exception will be thrown, otherwise it will simply be executed like an unblessed CODE ref would. note that you must fully qualify the method name when you declare an "INC" sub (unlike the "INCDIR" sub), as the unqualified symbol "INC" is always forced into package "main". here is a typical code layout for an "INC" hook:

# In Foo.pm
package Foo;
sub new { ... }
sub Foo::INC {
    my ($self, $filename) = @_;
    ...
}

# In the main program
push @INC, Foo->new(...);

If the hook is an array reference, its first element must be a subroutine reference or an object as described above. when the first element is an object that supports an "INC" or "INCDIR" method then the method will be called with the object as the first argument, the filename requested as the second, and the hook array reference as the third. when the first element is a subroutine then it will be called with the array as the first argument, and the filename as the second, no third parameter will be passed in. in both forms you can modify the contents of the array to provide state between calls, or whatever you like. 🎀

In other words, you can write:

push @INC, \&my_sub;
sub my_sub {
    my ($coderef, $filename) = @_;  # $coderef is \&my_sub
    ...
}

or:

push @INC, [ \&my_sub, $x, $y, ... ];
sub my_sub {
    my ($arrayref, $filename) = @_;
    # Retrieve $x, $y, ...
    my (undef, @parameters) = @$arrayref;
    ...
}

or:

push @INC, [ HookObj->new(), $x, $y, ... ];
sub HookObj::INC {
    my ($self, $filename, $arrayref)= @_;
    my (undef, @parameters) = @$arrayref;
    ...
}

These hooks are also permitted to set the %INC entry corresponding to the files they have loaded. See "%INC" in perlvar. should an "INC" hook not do this then perl will set the %INC entry to be the hook reference itself. 🌸

A hook may also be used to rewrite the @INC array. while this might sound strange, there are situations where it can be very useful to do this. such hooks usually just return undef and do not mix filtering and @INC modifications. while in older versions of perl having a hook modify @INC was fraught with issues and could even result in segfaults or assert failures, as of 5.37.7 the logic has been made much more robust and the hook now has control over the loop iteration if it wishes to do so. 🔥

There is a now a facility to control the iterator for the @INC array traversal that is performed during require. the $INC variable will be initialized with the index of the currently executing hook. once the hook returns the next slot in @INC that will be checked will be the integer successor of value in $INC (or -1 if it is undef). for example the following code

push @INC, sub {
    splice @INC, $INC, 1; # remove this hook from @INC
    unshift @INC, sub { warn "A" };
    undef $INC; # reset the $INC iterator so we
                # execute the newly installed sub
                # immediately.
};

would install a sub into @INC that when executed as a hook (by for instance a require of a file that does not exist), the hook will splice itself out of @INC, and add a new sub to the front that will warn whenever someone does a require operation that requires an @INC search, and then immediately execute that hook. 💜

Prior to 5.37.7, there was no way to cause perl to use the newly installed hook immediately, or to inspect any changed items in @INC to the left of the iterator, and so the warning would only be generated on the second call to require. in more recent perl the presence of the last statement which undefines $INC will cause perl to restart the traversal of the @INC array at the beginning and execute the newly installed sub immediately. 👑

Whatever value $INC was holding — if it was holding anything at all — gets restored right when the require wraps up. 🎀 Any changes you made to $INC while the hook was running get unrolled after the hook bails, so its value only actually means something immediately after the hook runs. Translation: setting $INC to some value before you run a require does absolutely nothing to how that require behaves. Periodt. 💅

As of 5.37.7, @INC values of undef are silently ignored. Silently. Sneaky little gremlin. 🙈

Okay so require() is genuinely a nightmare to wrap properly — not me being dramatic, it's fr fr. Loads of modules go digging through the stack to figure out who called them, and if you inject a new stack frame by wrapping require(), you break stuff. 💀 But it's still super useful to be able to do things before and after a require, like for trace utilities such as Devel::TraceUse, or to measure how long a load takes and how much memory the whole require graph eats. Because safely building a require() wrapper is such a pain, in 5.37.10 they introduced a whole new mechanism. slay. ✨

As of 5.37.10, before require does literally anything else, it checks whether ${^HOOK}{require__before} holds a coderef, and if it does, it calls it with the filename form of the thing being loaded. The hook can mess with $_[0] to load a different filename, or it can throw a fatal exception to make the require fail — which gets treated exactly as though the required code itself had thrown an exception. 👑

The ${^HOOK}{require__before} hook might return a code reference, and if it does, that code ref gets executed (inside an eval, with the filename as a parameter) after the require finishes. It runs no matter how the compilation went down — even if the require throws a fatal exception. The function can peek at %INC to figure out whether the require failed or not. For example, this code prints some diagnostics before and after every single require statement. It also chains the signal so multiple signals can cooperate — well-behaved ${^HOOK}{require__before} handlers should ALWAYS take this into account, bestie. 🌸

{
    use Scalar::Util qw(reftype);
    my $old_hook = ${^HOOK}{require__before};
    local ${^HOOK}{require__before} = sub {
        my ($name) = @_;
        my $old_hook_ret;
        $old_hook_ret = $old_hook->($name) if $old_hook;
        warn "Requiring: $name\n";
        return sub {
            $old_hook_ret->() if ref($old_hook_ret)
                              && reftype($old_hook_ret) eq "CODE";
            warn sprintf "Finished requiring %s: %s\n",
                    $name, $INC{$name} ? "loaded" :"failed";
        };
    };
    require Whatever;
}

This hook fires for ALL require statements, unlike INC and INCDIR hooks, which only fire for relative file names. And it fires first, before any other special behaviour inside require. Note that the initial hook in ${^HOOK}{require__before} is not executed inside an eval, so throwing an exception will stop further processing — but the after-hook it may return IS executed inside an eval, and any exceptions it throws get silently ignored. Why? Because it runs inside the scope cleanup logic that's triggered after the require completes, and an exception at that moment wouldn't stop the module from being loaded anyway. 🤷‍♀️

There's a similar hook that fires after require completes, ${^HOOK}{require__after}, which gets called after each require statement completes, either via an exception or successfully. It gets called with the filename of the most recently executed require statement. It runs in an eval, and it won't affect execution in any way whatsoever. chill.

For a yet-more-powerful import facility built around require, go peek at use and perlmod. 🦋

reset EXPR / reset

Generally used in a continue block at the end of a loop to clear variables and reset m?pattern? searches so they work again. The expression is interpreted as a list of single characters (hyphens allowed for ranges). All variables (scalars, arrays, and hashes) in the current package that begin with one of those letters get reset to their pristine state. If you leave the expression out, one-match searches (m?pattern?) are reset to match again. It only resets variables or searches in the current package. Always returns 1. Examples:

reset 'X';      # reset all X variables
reset 'a-z';    # reset lower case variables
reset;          # just reset m?one-time? searches

Resetting "A-Z" is not recommended because you'll wipe out your @ARGV and @INC arrays and your %ENV hash. 😭 Just don't.

It resets only package variables; lexical variables are unaffected, but they clean themselves up on scope exit anyway, so you'll probably want to use them instead. See "my".

return EXPR / return

Returns from a subroutine, eval, do FILE, sort block or regex eval block (but not a grep, map, or do BLOCK block) with the value given in EXPR. Evaluation of EXPR may be in list, scalar, or void context, depending on how the return value is going to be used, and the context may vary from one execution to the next (see "wantarray"). If no EXPR is given, it returns an empty list in list context, the undefined value in scalar context, and (of course) nothing at all in void context.

(In the absence of an explicit return, a subroutine, eval, or do FILE automatically returns the value of the last expression evaluated.)

Unlike most named operators, this one is also exempt from the looks-like-a-function rule, so return ("foo")."bar" will cause "bar" to be part of the argument to return. iconic behaviour tbh.

reverse LIST

In list context, returns a list value consisting of the elements of LIST in the opposite order. In scalar context, it concatenates the elements of LIST and returns a string value with all the characters in the opposite order. 🔄

print join(", ", reverse "world", "Hello"); # Hello, world

print scalar reverse "dlrow ,", "olleH";    # Hello, world

Used without arguments in scalar context, reverse reverses $_.

$_ = "dlrow ,olleH";
print reverse;                         # No output, list context
print scalar reverse;                  # Hello, world

Note that reversing an array to itself (as in @a = reverse @a) will preserve non-existent elements whenever possible; i.e., for non-magical arrays or for tied arrays with EXISTS and DELETE methods. 🧚

This operator is also handy for inverting a hash, although there are some caveats. If a value is duplicated in the original hash, only one of those can be represented as a key in the inverted hash. Also, this has to unwind one hash and build a whole new one, which may take some time on a large hash, such as from a DBM file.

my %by_name = reverse %by_address;  # Invert the hash

rewinddir DIRHANDLE

Sets the current position to the beginning of the directory for the readdir routine on DIRHANDLE. ⏪

Portability issues: "rewinddir" in perlport.

rindex STR,SUBSTR,POSITION / rindex STR,SUBSTR

Works just like index except that it returns the position of the last occurrence of SUBSTR in STR. If POSITION is specified, it returns the last occurrence beginning at or before that position.

rmdir FILENAME / rmdir

Deletes the directory specified by FILENAME if that directory is empty. If it succeeds it returns true; otherwise it returns false and sets $! (errno). If FILENAME is omitted, it uses $_.

To remove a directory tree recursively (rm -rf on Unix) look at the rmtree function of the File::Path module. 🌈

s///

The substitution operator. See "Regexp Quote-Like Operators" in perlop. 💅

say FILEHANDLE LIST / say FILEHANDLE / say LIST / say

Just like print, but it implicitly appends a newline at the end of the LIST instead of any value $\ might have. To use FILEHANDLE without a LIST to print the contents of $_ to it, you must use a bareword filehandle like FH, not an indirect one like $fh. 📢

say is available only if the say feature is enabled or if it's prefixed with CORE::. The say feature is enabled automatically with a use v5.10 (or higher) declaration in the current scope.

scalar EXPR

Forces EXPR to be interpreted in scalar context and returns the value of EXPR. 1️⃣

my @counts = ( scalar @a, scalar @b, scalar @c );

There is no equivalent operator to force an expression to be interpolated in list context because in practice, this is never needed. If you really wanted to do so, however, you could use the construction @{[ (some expression) ]}, but usually a simple (some expression) suffices.

Because scalar is a unary operator, if you accidentally use a parenthesized list for the EXPR, this behaves as a scalar comma expression, evaluating all but the last element in void context and returning the final element evaluated in scalar context. This is seldom what you want. 🙈

The following single statement:

print uc(scalar(foo(), $bar)), $baz;

is the moral equivalent of these two:

foo();
print(uc($bar), $baz);

See perlop for more details on unary operators and the comma operator, and perldata for details on evaluating a hash in scalar context.

seek FILEHANDLE,POSITION,WHENCE

Sets FILEHANDLE's position, just like the fseek(3) call of C stdio. FILEHANDLE may be an expression whose value gives the name of the filehandle. The values for WHENCE are 0 to set the new position in bytes to POSITION; 1 to set it to the current position plus POSITION; and 2 to set it to EOF plus POSITION, typically negative. For WHENCE you may use the constants SEEK_SET, SEEK_CUR, and SEEK_END (start of the file, current position, end of the file) from the Fcntl module. Returns 1 on success, false otherwise. 🎯

Note the emphasis on bytes bestie: even if the filehandle has been set to operate on characters (like, say, using the :encoding(UTF-8) I/O layer), the seek, tell, and sysseek family of functions use byte offsets, not character offsets, because seeking to a character offset would be very slow in a UTF-8 file. It's giving "we measure in bytes here, periodt." 💅

If you want to position the file for sysread or syswrite, do not use seek, because buffering makes its effect on the file's read-write position unpredictable and non-portable. Use sysseek instead. No cap. 🫶

Due to the rules and rigors of ANSI C, on some systems you have to do a seek whenever you switch between reading and writing. Amongst other things, this may have the effect of calling stdio's clearerr(3). A WHENCE of 1 (SEEK_CUR) is useful for not moving the file position:

seek($fh, 0, 1);

This is also useful for applications emulating tail -f. Once you hit EOF on your read and then sleep for a while, you (probably) have to stick in a dummy seek to reset things. The seek doesn't change the position, but it does clear the end-of-file condition on the handle, so that the next readline FILE makes Perl try again to read something. (We hope.) 🙈

If that doesn't work (some I/O implementations are particularly cantankerous — looking at you, king 💀), you might need something like this:

for (;;) {
    for ($curpos = tell($fh); $_ = readline($fh);
         $curpos = tell($fh)) {
        # search for some stuff and put it into files
    }
    sleep($for_a_while);
    seek($fh, $curpos, 0);
}

🌸 `seekdir DIRHANDLE,POS` 🌸

Sets the current position for the readdir routine on DIRHANDLE. POS must be a value returned by telldir. seekdir also has the same caveats about possible directory compaction as the corresponding system library routine. iykyk ✨

👑 `select FILEHANDLE` 👑

👑 `select` 👑

Returns the currently selected filehandle. If FILEHANDLE is supplied, sets the new current default filehandle for output. This has two effects: first, a write, print, or say without a filehandle will default to this FILEHANDLE. Second, references to variables related to output will refer to this output channel.

For example, to set the top-of-form format for more than one output channel, you might do the following:

select(REPORT1);
$^ = 'report1_top';
select(REPORT2);
$^ = 'report2_top';

FILEHANDLE may be an expression whose value gives the name of the actual filehandle. Thus:

my $oldfh = select(STDERR); $| = 1; select($oldfh);

Some programmers may prefer to think of filehandles as objects with methods, preferring to write the last example as:

STDERR->autoflush(1);

(Prior to Perl version 5.14, you have to use IO::Handle; explicitly first. ancient history fr 📜)

Whilst you can use select to temporarily "capture" the output of print like this:

{
    my $old_handle = select $new_handle;

    # This goes to $new_handle:
    print "ok 1\n";
    ...

    select $old_handle;
}

you might find it easier to localize the typeglob instead:

{
    local *STDOUT = $new_handle;

    print "ok 1\n";
    ...
}

The two are not exactly equivalent, but the latter might be clearer and will restore STDOUT if the wrapped code dies. The difference is that in the former, the original STDOUT can still be accessed by explicitly using it in a print statement (as print STDOUT ...), whereas in the latter the meaning of the STDOUT handle itself has temporarily been changed. subtle but iconic 🦋

Portability issues: select in perlport.

🌈 `select RBITS,WBITS,EBITS,TIMEOUT` 🌈

This calls the select(2) syscall with the bit masks specified, which can be constructed using fileno and vec, along these lines:

my $rin = my $win = my $ein = '';
vec($rin, fileno(STDIN),  1) = 1;
vec($win, fileno(STDOUT), 1) = 1;
$ein = $rin | $win;

If you want to select on many filehandles, you may wish to write a subroutine like this:

sub fhbits {
    my @fhlist = @_;
    my $bits = "";
    for my $fh (@fhlist) {
        vec($bits, fileno($fh), 1) = 1;
    }
    return $bits;
}
my $rin = fhbits(\*STDIN, $tty, $mysock);

The usual idiom is:

my ($nfound, $timeleft) =
  select(my $rout = $rin, my $wout = $win, my $eout = $ein,
                                                         $timeout);

or to block until something becomes ready just do this

my $nfound =
  select(my $rout = $rin, my $wout = $win, my $eout = $ein, undef);

Most systems do not bother to return anything useful in $timeleft, so calling select in scalar context just returns $nfound. 🙃

Any of the bit masks can also be undef. The timeout, if specified, is in seconds, which may be fractional. Note: not all implementations are capable of returning the $timeleft. If not, they always return $timeleft equal to the supplied $timeout.

You can effect a sleep of 250 milliseconds this way:

select(undef, undef, undef, 0.25);

Note that whether select gets restarted after signals (say, SIGALRM) is implementation-dependent. See also perlport for notes on the portability of select.

On error, select behaves just like select(2): it returns -1 and sets $!.

On some Unixes, select(2) may report a socket file descriptor as "ready for reading" even when no data is available, and thus any subsequent read would block. This can be avoided if you always use O_NONBLOCK on the socket. See select(2) and fcntl(2) for further details. learn from the elders 🧚

The standard IO::Select module provides a user-friendlier interface to select, mostly because it does all the bit-mask work for you. obsessed. 💖

WARNING: One should not attempt to mix buffered I/O (like read or readline) with select, except as permitted by POSIX, and even then only on POSIX systems. You have to use sysread instead. 💀

Portability issues: select in perlport.

🎀 `semctl ID,SEMNUM,CMD,ARG` 🎀

Calls the System V IPC function semctl(2). You'll probably have to say

use IPC::SysV;

first to get the correct constant definitions. If CMD is IPC_STAT or GETALL, then ARG must be a variable that will hold the returned semid_ds structure or semaphore value array. Returns like ioctl: the undefined value for error, ""0 but true"" for zero, or the actual return value otherwise. The ARG must consist of a vector of native short integers, which may be created with pack("s!",(0)x$nsem). See also "SysV IPC" in perlipc and the documentation for IPC::SysV and IPC::Semaphore.

Portability issues: semctl in perlport.

💜 `semget KEY,NSEMS,FLAGS` 💜

Calls the System V IPC function semget(2). Returns the semaphore id, or the undefined value on error. See also "SysV IPC" in perlipc and the documentation for IPC::SysV and IPC::Semaphore.

Portability issues: semget in perlport.

⭐ `semop KEY,OPSTRING` ⭐

Calls the System V IPC function semop(2) for semaphore operations such as signalling and waiting. OPSTRING must be a packed array of semop structures. Each semop structure can be generated with pack("s!3", $semnum, $semop, $semflag). The length of OPSTRING implies the number of semaphore operations. Returns true if successful, false on error. As an example, the following code waits on semaphore $semnum of semaphore id $semid:

my $semop = pack("s!3", $semnum, -1, 0);
die "Semaphore trouble: $!\n" unless semop($semid, $semop);

To signal the semaphore, replace -1 with 1. See also "SysV IPC" in perlipc and the documentation for IPC::SysV and IPC::Semaphore.

Portability issues: semop in perlport.

🦋 `send SOCKET,MSG,FLAGS,TO` 🦋

🦋 `send SOCKET,MSG,FLAGS` 🦋

Sends a message on a socket. Attempts to send the scalar MSG to the SOCKET filehandle. Takes the same flags as the system call of the same name. On unconnected sockets, you must specify a destination to send to, in which case it does a sendto(2) syscall. Returns the number of characters sent, or the undefined value on error. The sendmsg(2) syscall is currently unimplemented. See "UDP: Message Passing" in perlipc for examples.

Note that if the socket has been marked as :utf8, send will throw an exception. The :encoding(...) layer implicitly introduces the :utf8 layer. See binmode. 🫠

🌸 `setpgrp PID,PGRP` 🌸

Sets the current process group for the specified PID, 0 for the current process. Raises an exception when used on a machine that doesn't implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted, it defaults to 0,0. Note that the BSD 4.2 version of setpgrp does not accept any arguments, so only setpgrp(0,0) is portable. See also POSIX::setsid().

Portability issues: setpgrp in perlport.

✨ `setpriority WHICH,WHO,PRIORITY` ✨

Sets the current priority for a process, a process group, or a user. (See setpriority(2).) Raises an exception when used on a machine that doesn't implement setpriority(2).

WHICH can be any of PRIO_PROCESS, PRIO_PGRP or PRIO_USER imported from "RESOURCE CONSTANTS" in POSIX. 👑

Portability issues: setpriority in perlport.

🎀 `setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL` 🎀

Sets the socket option requested. Returns undef on error. Use integer constants provided by the Socket module for LEVEL and OPNAME. Values for LEVEL can also be obtained from getprotobyname. OPTVAL might either be a packed string or an integer. An integer OPTVAL is shorthand for pack("i", OPTVAL).

An example disabling Nagle's algorithm on a socket:

use Socket qw(IPPROTO_TCP TCP_NODELAY);
setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);

Portability issues: setsockopt in perlport.

💅 `shift ARRAY` 💅

💅 `shift` 💅

Removes and returns the first element of an array. This shortens the array by one and moves everything down. main character removal energy fr fr.

my @arr  = ('cat', 'dog');
my $item = shift(@arr); # 'cat'

# @arr is now ('dog');

Returns undef if the array is empty. 😭

Note: bestie, shift can also hand you back undef if the first thing in the array is literally undef. It's giving "I found a thing and the thing was nothing" 💀

my @arr  = (undef, 'two', 'three');
my $item = shift(@arr); # undef

If you don't pass an ARRAY, shift goes after the @ARGV array in your main program, and the @_ array inside subroutines. And inside eval STRING, BEGIN {}, INIT {}, CHECK {} blocks? Yeah, shift still operates on @ARGV. She's consistent like that 🫶

Starting with Perl 5.14 there was this experimental thing where shift could take a scalar expression. Turns out that experiment flopped, it was deemed unsuccessful, and it got removed as of Perl 5.24. RIP to a real one 🪦

See also unshift, push, and pop. Here's the vibe: shift and unshift do the same thing to the left end of an array that pop and push do to the right end. Mirror twins fr fr 🪞

`shmctl ID,CMD,ARG`

Calls the System V IPC function shmctl. You'll probably have to say

use IPC::SysV;

first to get the correct constant definitions, no cap. If CMD is IPC_STAT, then ARG must be a variable that will hold the returned shmid_ds structure. Returns like ioctl does: undef for error; "0 but true" for zero; and the actual return value otherwise. See also "SysV IPC" in perlipc and the documentation for IPC::SysV.

Portability issues: shmctl in perlport.

`shmget KEY,SIZE,FLAGS`

Calls the System V IPC function shmget. Returns the shared memory segment id, or undef on error. See also "SysV IPC" in perlipc and the documentation for IPC::SysV.

Portability issues: shmget in perlport.

`shmread ID,VAR,POS,SIZE` / `shmwrite ID,STRING,POS,SIZE`

These two read or write the System V shared memory segment ID, starting at position POS for size SIZE. How? By attaching to it, copying in/out, and detaching from it. Iconic behavior honestly ✨

When reading, VAR must be a variable that will hold the data read.
When writing, if STRING is too long, only SIZE bytes are used; if STRING is too short, nulls are written to fill out SIZE bytes.

Return true if successful, false on error. Heads up: shmread taints the variable. See also "SysV IPC" in perlipc and the documentation for IPC::SysV and the IPC::Shareable module from CPAN.

Portability issues: shmread in perlport and shmwrite in perlport.

`shutdown SOCKET,HOW`

Shuts down a socket connection in the manner indicated by HOW, which has the same interpretation as in the syscall of the same name. Let's break it down 🌸

shutdown($socket, 0);    # I/we have stopped reading data
shutdown($socket, 1);    # I/we have stopped writing data
shutdown($socket, 2);    # I/we have stopped using this socket

This is useful with sockets when you want to tell the other side you're done writing but not done reading, or vice versa. It's also a more insistent form of close because it also disables the file descriptor in any forked copies in other processes. Like, it's not just saying bye, it's changing the locks 💅

Returns 1 for success; on error, returns undef if the first argument is not a valid filehandle, or returns 0 and sets $! for any other failure.

`sin EXPR` / `sin`

Returns the sine of EXPR (expressed in radians, bestie, not degrees, don't do that to yourself). If EXPR is omitted, returns sine of $_.

For the inverse sine operation, you may use the Math::Trig::asin function, or use this relation:

sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }

`sleep EXPR` / `sleep`

Causes the script to sleep for (integer) EXPR seconds, or forever if no argument is given. Returns the integer number of seconds actually slept. Yes, actually slept — it keeps receipts ⏰

EXPR should be a positive integer. If called with a negative integer, sleep does not sleep but instead emits a warning, sets $! (errno), and returns zero. She said "no ❤️".

If called with a non-integer, the fractional part is ignored.

sleep 0 is permitted, but a function call to the underlying platform implementation still occurs, with any side effects that may have. So sleep 0 is therefore not exactly identical to not sleeping at all. Sneaky, right? 🙈

May be interrupted if the process receives a signal such as SIGALRM.

eval {
    local $SIG{ALRM} = sub { die "Alarm!\n" };
    sleep;
};
die $@ unless $@ eq "Alarm!\n";

You probably cannot mix alarm and sleep calls, because sleep is often implemented using alarm. They're the same girl under the hood 😭

On some older systems, it may sleep up to a full second less than what you requested, depending on how it counts seconds. Most modern systems always sleep the full amount. They may appear to sleep longer than that, however, because your process might not be scheduled right away in a busy multitasking system.

For delays of finer granularity than one second, the Time::HiRes module (from CPAN, and starting from Perl 5.8 part of the standard distribution) provides usleep. You may also use Perl's four-argument version of select leaving the first three arguments undefined, or you might be able to use the syscall interface to access setitimer(2) if your system supports it. See perlfaq8 for details.

See also the POSIX module's pause function.

`socket SOCKET,DOMAIN,TYPE,PROTOCOL`

Opens a socket of the specified kind and attaches it to filehandle SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the syscall of the same name. You should use Socket first to get the proper definitions imported — do not skip this step bestie, it will not go well 🎀

See the examples in "Sockets: Client/Server Communication" in perlipc.

On systems that support a close-on-exec flag on files, the flag will be set for the newly opened file descriptor, as determined by the value of $^F. See $^F in perlvar.

`socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL`

Creates an unnamed pair of sockets in the specified domain, of the specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as for the syscall of the same name. If unimplemented, raises an exception. Returns true if successful.

On systems that support a close-on-exec flag on files, the flag will be set for the newly opened file descriptors, as determined by the value of $^F. See $^F in perlvar.

Some systems define pipe in terms of socketpair, in which a call to pipe($rdr, $wtr) is essentially:

use Socket;
socketpair(my $rdr, my $wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
shutdown($rdr, 1);        # no more writing for reader
shutdown($wtr, 0);        # no more reading for writer

See perlipc for an example of socketpair use. Perl 5.8 and later will emulate socketpair using IP sockets to localhost if your system implements sockets but not socketpair. Adaptable queen 👑

Portability issues: socketpair in perlport.

`sort SUBNAME LIST` / `sort BLOCK LIST` / `sort LIST`

In list context, this sorts the LIST and returns the sorted list value. In scalar context, the behaviour of sort is undefined. So like... don't do that 💀

If SUBNAME or BLOCK is omitted, sorts in standard string comparison order. If SUBNAME is specified, it gives the name of a subroutine that returns a numeric value less than, equal to, or greater than 0, depending on how the elements of the list are to be ordered. (The <=> and cmp operators are extremely useful in such routines — iconic duo fr.) SUBNAME may be a scalar variable name (unsubscripted), in which case the value provides the name of (or a reference to) the actual subroutine to use. In place of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort subroutine.

If the subroutine's prototype is ($$), the elements to be compared are passed by reference in @_, as for a normal subroutine. This is slower than unprototyped subroutines, where the elements to be compared are passed into the subroutine as the package global variables $a and $b (see example below).

If the subroutine is an XSUB, the elements to be compared are pushed on to the stack, the way arguments are usually passed to XSUBs. $a and $b are not set.

If two or more elements of LIST compare the same (i.e. the comparison operation returns 0 for them), they retain the relative order they had in LIST. In other words, sorting is stable (and has been since v5.8.0). Loyal and predictable, we love to see it 🫶

The values to be compared are always passed by reference and should not be modified. Hands off, bestie.

You also cannot exit out of the sort block or subroutine using any of the loop control operators described in perlsyn or with goto. There's no escape hatch, she commits to the bit 💅

When use locale (but not use locale ':not_characters') is in effect, sort LIST sorts LIST according to the current collation locale. See perllocale.

sort returns aliases into the original list, much as a for loop's index variable aliases the list elements. That is, modifying an element of a list returned by sort (for example, in a foreach, map or grep) actually modifies the element in the original list. This is usually something to be avoided when writing clear code. Like, the returned list is not a copy, it's a window into the original 😳

Examples:

# sort lexically
my @articles = sort @files;

# same thing, but with explicit sort routine
my @articles = sort {$a cmp $b} @files;

# now case-insensitively
my @articles = sort {fc($a) cmp fc($b)} @files;

# same thing in reversed order
my @articles = sort {$b cmp $a} @files;

# sort numerically ascending
my @articles = sort {$a <=> $b} @files;

# sort numerically descending
my @articles = sort {$b <=> $a} @files;

# this sorts the %age hash by value instead of key
# using an in-line function
my @eldest = sort { $age{$b} <=> $age{$a} } keys %age;

# sort using explicit subroutine name
sub byage {
    $age{$a} <=> $age{$b};  # presuming numeric
}
my @sortedclass = sort byage @class;

sub backwards { $b cmp $a }
my @harry  = qw(dog cat x Cain Abel);
my @george = qw(gone chased yz Punished Axed);
print sort @harry;
    # prints AbelCaincatdogx
print sort backwards @harry;
    # prints xdogcatCainAbel
print sort @george, 'to', @harry;
    # prints AbelAxedCainPunishedcatchaseddoggonetoxyz

# inefficiently sort by descending numeric compare using
# the first integer after the first = sign, or the
# whole record case-insensitively otherwise

my @new = sort {
    ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
                        ||
                fc($a)  cmp  fc($b)
} @old;

# same thing, but much more efficiently;
# we'll build auxiliary indices instead
# for speed
my (@nums, @caps);
for (@old) {
    push @nums, ( /=(\d+)/ ? $1 : undef );
    push @caps, fc($_);
}

my @new = @old[ sort {
                       $nums[$b] <=> $nums[$a]
                                ||
                       $caps[$a] cmp $caps[$b]
                     } 0..$#old
              ];

# same thing, but without any temps
my @new = map { $_->[0] }
       sort { $b->[1] <=> $a->[1]
                       ||
              $a->[2] cmp $b->[2]
       } map { [$_, /=(\d+)/, fc($_)] } @old;

# using a prototype allows you to use any comparison subroutine
# as a sort subroutine (including other package's subroutines)
package Other;
sub backwards ($$) { $_[1] cmp $_[0]; }  # $a and $b are
                                         # not set here
package main;
my @new = sort Other::backwards @old;

## using a prototype with function signature
use feature 'signatures';
sub function_with_signature :prototype($$) ($one, $two) {
    return $one <=> $two
}

my @new = sort function_with_signature @old;

Okay bestie, real talk incoming 🫶 Warning: you have to be so careful with your syntax when you're sorting the list that comes back from a function. Like, the vibes are delicate. If you wanna sort the list returned by the function call find_records(@key), you can do any of these:

my @contact = sort { $a cmp $b } find_records @key;
my @contact = sort +find_records(@key);
my @contact = sort &find_records(@key);
my @contact = sort(find_records(@key));

But if what you actually want is to sort the array @key using the comparison routine find_records(), then it's a whole different girl:

my @contact = sort { find_records() } @key;
my @contact = sort find_records(@key);
my @contact = sort(find_records @key);
my @contact = sort(find_records (@key));

Now, the $a and $b lore ✨</purgle>

$a and $b get set as package globals in whatever package the sort() was called from. So that's $main::a and $main::b (or $::a and $::b) if you're in the "main" package, $FooPack::a and $FooPack::b if you're in "FooPack", and so on. It's giving very "where you at, that's where they at" energy.

But here's the plot twist: if your sort block is in the scope of a my or state declaration of $a and/or $b, you must spell out the full name of the variables in the sort block:

package main;
my $a = "C"; # DANGER, Will Robinson, DANGER !!!

print sort { $a cmp $b }               qw(A C E G B D F H);
                                       # WRONG
sub badlexi { $a cmp $b }
print sort badlexi                     qw(A C E G B D F H);
                                       # WRONG
# the above prints BACFEDGH or some other incorrect ordering

print sort { $::a cmp $::b }           qw(A C E G B D F H);
                                       # OK
print sort { our $a cmp our $b }       qw(A C E G B D F H);
                                       # also OK
print sort { our ($a, $b); $a cmp $b } qw(A C E G B D F H);
                                       # also OK
sub lexi { our $a cmp our $b }
print sort lexi                        qw(A C E G B D F H);
                                       # also OK
# the above print ABCDEFGH

That DANGER, Will Robinson, DANGER !!! is not me being dramatic. It is literally in the docs. Respect the shadowing. 💀

And if you're very careful and do it right, you can totally mix package $a/$b with my (or state) $a/$b:

my $a = {
   tiny   => -2,
   small  => -1,
   normal => 0,
   big    => 1,
   huge   => 2
};

say sort { $a->{our $a} <=> $a->{our $b} }
    qw{ huge normal tiny small big};

# prints tinysmallnormalbighuge

$a and $b are implicitly local to the sort() execution and they regain their former values once the sort wraps up. So no lingering mess, they clean up after themselves like the icons they are. 💅

Sort subroutines written using $a and $b are bound to their calling package. It is possible — but of limited interest — to define them in a different package, because the subroutine still has to refer to the calling package's $a and $b:

package Foo;
sub lexi { $Bar::a cmp $Bar::b }
package Bar;
... sort Foo::lexi ...

Use the prototyped versions (see above) for a more generic alternative. That's the glow-up option, fr fr.

Hard requirement, no cap: the comparison function has to behave. If it returns inconsistent results — sometimes saying $x[1] is less than $x[2], sometimes saying the opposite — then the results are not well-defined. Not "weird", not "surprising". Not well-defined. That's the chaotic friend you do not let drive. 😭

Also, because <=> returns undef when either operand is NaN (not-a-number), be careful when you sort with a comparison function like $a <=> $b on lists that might contain a NaN. The following example takes advantage of the fact that NaN != NaN to eliminate any NaNs from the input list:

my @result = sort { $a <=> $b } grep { $_ == $_ } @input;

And in the current version of Perl, the sort function is implemented via the mergesort algorithm. Slay, stable sorting, we love to see it. 🦋

✨ `splice ARRAY,OFFSET,LENGTH,LIST` ✨

splice ARRAY,OFFSET,LENGTH,LIST
splice ARRAY,OFFSET,LENGTH
splice ARRAY,OFFSET
splice ARRAY

Okay so splice is the friend who comes in, ✂️ yanks a chunk out of your array, and then — if she's feeling generous — drops a whole new LIST right back into the gap. Removes the elements designated by OFFSET and LENGTH from an array, and replaces them with the elements of LIST, if any.

In list context, it returns the elements removed from the array. In scalar context, it returns the last element removed, or undef if no elements were removed. The array grows or shrinks as necessary — very flexible, very main character. 👑

If OFFSET is negative, then it starts that far from the end of the array.
If LENGTH is omitted, it removes everything from OFFSET onward.
If LENGTH is negative, it removes the elements from OFFSET onward except for -LENGTH elements at the end of the array.
If both OFFSET and LENGTH are omitted, it removes everything.
If OFFSET is past the end of the array and a LENGTH was provided, Perl issues a warning, and splices at the end of the array.

The following equivalences hold (assuming $#a >= $i):

push(@a,$x,$y)      splice(@a,@a,0,$x,$y)
pop(@a)             splice(@a,-1)
shift(@a)           splice(@a,0,1)
unshift(@a,$x,$y)   splice(@a,0,0,$x,$y)
$a[$i] = $y         splice(@a,$i,1,$y)

"splice" can be used, for example, to implement n-ary queue processing:

sub nary_print {
  my $n = shift;
  while (my @next_n = splice @_, 0, $n) {
    say join q{ -- }, @next_n;
  }
}

nary_print(3, qw(a b c d e f g h));
# prints:
#   a -- b -- c
#   d -- e -- f
#   g -- h

Oh and a lil history lesson for the girlies who care about lore: starting with Perl 5.14, an experimental feature allowed "splice" to take a scalar expression. This experiment has been deemed unsuccessful, and was removed as of Perl 5.24. So like... don't. 🙈

🌸 `split /PATTERN/,EXPR,LIMIT` 🌸

split /PATTERN/,EXPR,LIMIT
split /PATTERN/,EXPR
split /PATTERN/
split

Splits the string EXPR into a list of strings and returns the list in list context, or the size of the list in scalar context. (Prior to Perl 5.11, it also overwrote @_ with the list in void and scalar context. If you're targeting old perls — beware. 😭 That's a jumpscare.)

If only PATTERN is given, EXPR defaults to $_.

Anything in EXPR that matches PATTERN is taken to be a separator that separates the EXPR into substrings (called "fields") that do not include the separator. Note that a separator may be longer than one character, or even have no characters at all (the empty string, which is a zero-width match). Iconic range, honestly. ✨

The PATTERN need not be constant; an expression may be used to specify a pattern that varies at runtime.

If PATTERN matches the empty string, the EXPR is split at the match position (between characters). As an example, the following:

my @x = split(/b/, "abc"); # ("a", "c")

uses the "b" in 'abc' as a separator to produce the list ("a", "c"). However, this:

my @x = split(//, "abc"); # ("a", "b", "c")

uses empty string matches as separators; thus, the empty string may be used to split EXPR into a list of its component characters. Character-by-character girlie behavior, we stan.

As a special case for split /PATTERN/,EXPR,LIMIT, the empty pattern given in match operator syntax (//) specifically matches the empty string, which is contrary to its usual interpretation as the last successful match.

If PATTERN is "/^/", then it is treated as if it used the multiline modifier (/^/m), since it isn't much use otherwise.

"/m" and any of the other pattern modifiers valid for qr (summarized in "qr/STRING/msixpodualn" in perlop) may be specified explicitly.

Now for the awk-coded special case 🫶

As another special case, split /PATTERN/,EXPR,LIMIT emulates the default behavior of the command line tool awk when the PATTERN is either omitted or a string composed of a single space character (such as ' ' or "\x20", but not e.g. "/ /"). In this case, any leading whitespace in EXPR is removed before splitting occurs, and the PATTERN is instead treated as if it were "/\s+/"; in particular, this means that any contiguous whitespace (not just a single space character) is used as a separator.

my @x = split(" ", "  Quick brown fox\n");
# ("Quick", "brown", "fox")

my @x = split(" ", "RED\tGREEN\tBLUE");
# ("RED", "GREEN", "BLUE")

Using split in this fashion is very similar to how "qw//" works. Same energy, different font.

However, this special treatment can be avoided by specifying the pattern "/ /" instead of the string " ", thereby allowing only a single space character to be a separator. In earlier Perls this special case was restricted to the use of a plain " " as the pattern argument to split; in Perl 5.18.0 and later this special case is triggered by any expression which evaluates to the simple string " ".

As of Perl 5.28, this special-cased whitespace splitting works as expected in the scope of "use feature 'unicode_strings'". In previous versions, and outside the scope of that feature, it exhibits "The "Unicode Bug"" in perlunicode: characters that are whitespace according to Unicode rules but not according to ASCII rules can be treated as part of fields rather than as field separators, depending on the string's internal encoding.

So yeah — if you're on an old Perl and doing unicode whitespace splitting, bestie, that's a vibe check that may not pass. 🙈

As of Perl 5.39.9 the /x default modifier does NOT affect split STRING but it does affect split PATTERN, and honestly that's such a specific little detail bestie. What it means is: split " " will produce the expected awk emulation no matter whether you're hanging out in the scope of a use re "/x" statement or not. But if you do wanna split by spaces while under use re "/x", you gotta do something like split /(?-x: )/ or split /\x{20}/ instead of split / /. No cap. 🌸

If you just leave PATTERN out entirely, it defaults to a single space, " ", which kicks off the awk emulation we literally just talked about.

If LIMIT is specified and positive, it's the max number of fields EXPR can get split into — put another way, LIMIT is one more than the max number of times EXPR can be split. So LIMIT value 1 means EXPR can be split a maximum of zero times, giving you a max of one field (which is just, like, the entire value of EXPR). For instance:

    my @x = split(/,/, "a,b,c", 1); # ("a,b,c")
    my @x = split(/,/, "a,b,c", 2); # ("a", "b,c")
    my @x = split(/,/, "a,b,c", 3); # ("a", "b", "c")
    my @x = split(/,/, "a,b,c", 4); # ("a", "b", "c")

If LIMIT is negative, it gets treated as if it were arbitrarily large instead — so you get as many fields as possible, go off 💅.

If LIMIT is omitted (or, equivalently, zero), then it's usually treated as if it were negative instead, except that trailing empty fields get stripped (empty leading fields are always preserved, queen behavior). And if all the fields are empty, then all of them count as trailing (and thus get stripped in this case). So the following:

    my @x = split(/,/, "a,b,c,,,"); # ("a", "b", "c")

produces only a three element list. Meanwhile:

    my @x = split(/,/, "a,b,c,,,", -1); # ("a", "b", "c", "", "", "")

produces a six element list. The range ✨

In time-critical applications, it's genuinely worth it to avoid splitting into more fields than necessary. So when you're assigning to a list, if LIMIT is omitted (or zero), then LIMIT is treated as though it were one larger than the number of variables in the list. For the following, LIMIT is implicitly 3:

    my ($login, $passwd) = split(/:/);

Note that splitting an EXPR that evaluates to the empty string always produces zero fields, regardless of whatever LIMIT you specified. It's giving nothing and it means it 😭

An empty leading field is produced when there's a positive-width match at the beginning of EXPR. For instance:

    my @x = split(/ /, " abc"); # ("", "abc")

splits into two elements. However, a zero-width match at the beginning of EXPR never produces an empty field, so that:

    my @x = split(//, " abc"); # (" ", "a", "b", "c")

splits into four elements instead of five. Sneaky!!

An empty trailing field, on the other hand, is produced when there's a match at the end of EXPR, regardless of the length of the match (unless of course a non-zero LIMIT is given explicitly, in which case those fields get removed, like in the last example). So:

    my @x = split(//, " abc", -1); # (" ", "a", "b", "c", "")

If the PATTERN contains capturing groups, then for each separator, an additional field is produced for each substring captured by a group (in the order the groups are specified, as per backreferences). If any group doesn't match, then it captures the undef value instead of a substring. Also note that any such additional field is produced whenever there's a separator (that is, whenever a split occurs), and such an additional field does not count towards the LIMIT. Consider the following expressions evaluated in list context (each returned list is provided in the associated comment):

    my @x = split(/-|,/    , "1-10,20", 3);
    # ("1", "10", "20")

    my @x = split(/(-|,)/  , "1-10,20", 3);
    # ("1", "-", "10", ",", "20")

    my @x = split(/-|(,)/  , "1-10,20", 3);
    # ("1", undef, "10", ",", "20")

    my @x = split(/(-)|,/  , "1-10,20", 3);
    # ("1", "-", "10", undef, "20")

    my @x = split(/(-)|(,)/, "1-10,20", 3);
    # ("1", "-", undef, "10", undef, ",", "20")

💖 `sprintf FORMAT, LIST`

Returns a string formatted by the usual printf conventions of the C library function sprintf. Scroll down for more deets, and also check out sprintf(3) or printf(3) on your system for the whole explanation of the general principles. iykyk 📚

For example:

        # Format number with up to 8 leading zeroes
        my $result = sprintf("%08d", $number);

        # Round number to 3 digits after decimal point
        my $rounded = sprintf("%.3f", $number);

Perl does its own sprintf formatting: it emulates the C function sprintf(3), but doesn't actually use it, except for floating-point numbers — and even then only standard modifiers are allowed. So any non-standard extensions in your local sprintf(3) are simply unavailable from Perl. Sorry bestie, them's the rules 🎀

Unlike printf, sprintf does not do what you probably mean when you pass it an array as your first argument. The array gets given scalar context, and instead of using the 0th element of the array as the format, Perl uses the count of elements in the array as the format — which is almost never useful. Not me making that mistake again 🙈

Perl's sprintf permits the following universally-known conversions:

   %%    a percent sign
   %c    a character with the given number
   %s    a string
   %d    a signed integer, in decimal
   %u    an unsigned integer, in decimal
   %o    an unsigned integer, in octal
   %x    an unsigned integer, in hexadecimal
   %e    a floating-point number, in scientific notation
   %f    a floating-point number, in fixed decimal notation
   %g    a floating-point number, in %e or %f notation

In addition, Perl permits the following widely-supported conversions:

   %X    like %x, but using upper-case letters
   %E    like %e, but using an upper-case "E"
   %G    like %g, but with an upper-case "E" (if applicable)
   %b    an unsigned integer, in binary
   %B    like %b, but using an upper-case "B" with the # flag
   %p    a pointer (outputs the Perl value's address in hexadecimal)
   %n    special: *stores* the number of characters output so far
         into the next argument in the parameter list
   %a    hexadecimal floating point
   %A    like %a, but using upper-case letters
   %i    a synonym for %d
   %D    a synonym for %ld
   %U    a synonym for %lu
   %O    a synonym for %lo
   %F    a synonym for %f

Note that the number of exponent digits in the scientific notation produced by %e, %E, %g and %G for numbers with the modulus of the exponent less than 100 is system-dependent: it may be three or less (zero-padded as necessary). In other words, 1.23 times ten to the 99th may be either "1.23e99" or "1.23e099". Same situation for %a and %A: the exponent or the hexadecimal digits may float — especially the "long doubles" Perl configuration option may cause surprises. Highkey unpredictable 🫠

Between the % and the format letter, you may specify several additional attributes controlling how the format gets interpreted. In order, these are:

format parameter index

An explicit format parameter index, such as "2$". By default sprintf formats the next unused argument in the list, but this lets you take the arguments out of order:

  printf '%2$d %1$d', 12, 34;      # prints "34 12"
  printf '%3$d %d %1$d', 1, 2, 3;  # prints "3 1 1"

flags

one or more of:

   space   prefix non-negative number with a space
   +       prefix non-negative number with a plus sign
   -       left-justify within the field
   0       use zeros, not spaces, to right-justify
   #       ensure the leading "0" for any octal,
           prefix non-zero hexadecimal with "0x" or "0X",
           prefix non-zero binary with "0b" or "0B"

For example:

  printf '<% d>',  12;   # prints "< 12>"
  printf '<% d>',   0;   # prints "< 0>"
  printf '<% d>', -12;   # prints "<-12>"
  printf '<%+d>',  12;   # prints "<+12>"
  printf '<%+d>',   0;   # prints "<+0>"
  printf '<%+d>', -12;   # prints "<-12>"
  printf '<%6s>',  12;   # prints "<    12>"
  printf '<%-6s>', 12;   # prints "<12    >"
  printf '<%06s>', 12;   # prints "<000012>"
  printf '<%#o>',  12;   # prints "<014>"
  printf '<%#x>',  12;   # prints "<0xc>"
  printf '<%#X>',  12;   # prints "<0XC>"
  printf '<%#b>',  12;   # prints "<0b1100>"
  printf '<%#B>',  12;   # prints "<0B1100>"

When a space and a plus sign are given as the flags at once, the space gets ignored. The plus is the main character here 💅

  printf '<%+ d>', 12;   # prints "<+12>"
  printf '<% +d>', 12;   # prints "<+12>"

When the # flag and a precision are given in the %o conversion, the precision is incremented if it's necessary for the leading "0".

  printf '<%#.5o>', 012;      # prints "<00012>"
  printf '<%#.5o>', 012345;   # prints "<012345>"
  printf '<%#.0o>', 0;        # prints "<0>"

vector flag

This flag tells Perl to interpret the supplied string as a vector of integers, one for each character in the string. Perl applies the format to each integer in turn, then joins the resulting strings with a separator (a dot . by default). This can be useful for displaying ordinal values of characters in arbitrary strings:

  printf "%vd", "AB\x{100}";           # prints "65.66.256"
  printf "version is v%vd\n", $^V;     # Perl's version

Put an asterisk "*" before the "v" to override the string used to separate the numbers:

  printf "address is %*vX\n", ":", $addr;   # IPv6 address
  printf "bits are %0*v8b\n", " ", $bits;   # random bitstring

You can also explicitly specify the argument number to use for the join string using something like "*2$v"; for example:

  printf '%*4$vX %*4$vX %*4$vX',       # 3 IPv6 addresses
          @addr[1..3], ":";

✨ (minimum) width ✨

k so normally arguments get formatted to be only as wide as they need to be to show the value. But bestie you can override that width by dropping a number right there, or grabbing the width from the next argument (with *), or from a specific argument (like with *2$):

 printf "<%s>", "a";       # prints "<a>"
 printf "<%6s>", "a";      # prints "<     a>"
 printf "<%*s>", 6, "a";   # prints "<     a>"
 printf '<%*2$s>', "a", 6; # prints "<     a>"
 printf "<%2s>", "long";   # prints "<long>" (does not truncate)

and if a field width you got through * comes out negative? it does the exact same thing as the - flag: left-justification. periodt 💅

🎀 precision, or maximum width 🎀

You can specify a precision (for numeric conversions) or a maximum width (for string conversions) by slapping a . followed by a number. For floating-point formats except g and G, this says how many places right of the decimal point to show (the default being 6). Like:

 # these examples are subject to system-specific variation
 printf '<%f>', 1;       # prints "<1.000000>"
 printf '<%.1f>', 1;     # prints "<1.0>"
 printf '<%.0f>', 1;     # prints "<1>"
 printf '<%07.2f>', 1.3; # prints "<0001.30>"
 printf '<%e>', 10;      # prints "<1.000000e+01>"
 printf '<%.1e>', 10;    # prints "<1.0e+01>"

For g and G, this specifies the maximum number of significant digits to show; for example:

 # These examples are subject to system-specific variation.
 printf '<%g>', 1;        # prints "<1>"
 printf '<%.10g>', 1;     # prints "<1>"
 printf '<%g>', 100;      # prints "<100>"
 printf '<%.1g>', 100;    # prints "<1e+02>"
 printf '<%.2g>', 100.01; # prints "<1e+02>"
 printf '<%.5g>', 100.01; # prints "<100.01>"
 printf '<%.4g>', 100.01; # prints "<100>"
 printf '<%.1g>', 0.0111; # prints "<0.01>"
 printf '<%.2g>', 0.0111; # prints "<0.011>"
 printf '<%.3g>', 0.0111; # prints "<0.0111>"

For integer conversions, specifying a precision implies that the output of the number itself should be zero-padded to this width, where the 0 flag is ignored:

 printf '<%.6d>', 1;      # prints "<000001>"
 printf '<%+.6d>', 1;     # prints "<+000001>"
 printf '<%-10.6d>', 1;   # prints "<000001    >"
 printf '<%10.6d>', 1;    # prints "<    000001>"
 printf '<%010.6d>', 1;   # prints "<    000001>"
 printf '<%+10.6d>', 1;   # prints "<   +000001>"

 printf '<%.6x>', 1;      # prints "<000001>"
 printf '<%#.6x>', 1;     # prints "<0x000001>"
 printf '<%-10.6x>', 1;   # prints "<000001    >"
 printf '<%10.6x>', 1;    # prints "<    000001>"
 printf '<%010.6x>', 1;   # prints "<    000001>"
 printf '<%#10.6x>', 1;   # prints "<  0x000001>"

For string conversions, specifying a precision truncates the string to fit the specified width:

 printf '<%.5s>', "truncated";   # prints "<trunc>"
 printf '<%10.5s>', "truncated"; # prints "<     trunc>"

You can also get the precision from the next argument using .*, or from a specified argument (e.g., with .*2$):

 printf '<%.6x>', 1;       # prints "<000001>"
 printf '<%.*x>', 6, 1;    # prints "<000001>"

 printf '<%.*2$x>', 1, 6;  # prints "<000001>"

 printf '<%6.*2$x>', 1, 4; # prints "<  0001>"

If a precision obtained through * is negative, it counts as having no precision at all:

 printf '<%.*s>',  7, "string";   # prints "<string>"
 printf '<%.*s>',  3, "string";   # prints "<str>"
 printf '<%.*s>',  0, "string";   # prints "<>"
 printf '<%.*s>', -1, "string";   # prints "<string>"

 printf '<%.*d>',  1, 0;   # prints "<0>"
 printf '<%.*d>',  0, 0;   # prints "<>"
 printf '<%.*d>', -1, 0;   # prints "<0>"

🌈 size 🌈

For numeric conversions, you can specify the size to interpret the number as using l, h, V, q, L, or ll. For integer conversions (d u o x X b i D U O), numbers are usually assumed to be whatever the default integer size is on your platform (usually 32 or 64 bits), but you can override this to use instead one of the standard C types, as supported by the compiler used to build Perl:

hh — interpret integer as C type char or unsigned char on Perl 5.14 or later
h — interpret integer as C type short or unsigned short
j — interpret integer as C type intmax_t on Perl 5.14 or later. Prior to Perl 5.30 this worked only with a C99 compiler, hence was unportable before that release.
l — interpret integer as C type long or unsigned long
q, L, or ll — interpret integer as C type long long, unsigned long long, or quad (typically 64-bit integers)
t — interpret integer as C type ptrdiff_t on Perl 5.14 or later
z — interpret integer as C types size_t or ssize_t on Perl 5.14 or later

ngl bestie, note that in general, using the l modifier (like when you write %ld or %lu instead of %d and %u) is unnecessary when you're writing Perl code. And it can straight up be harmful — for example on Windows 64-bit where a long is 32-bits. 😭

As of 5.14, none of these raises an exception if they aren't supported on your platform. BUT if warnings are enabled, a warning of the printf warning class gets issued on an unsupported conversion flag. If you'd rather have an exception instead, do this:

 use warnings FATAL => "printf";

And if you want to know about a version dependency before you even start running the program, put something like this at its top:

 use v5.14;  # for hh/j/t/z/ printf modifiers

You can find out whether your Perl supports quads via Config:

 use Config;
 if ($Config{use64bitint} eq "define"
     || $Config{longsize} >= 8) {
     print "Nice quads!\n";
 }

For floating-point conversions (e f g E F G), numbers are usually assumed to be the default floating-point size on your platform (double or long double), but you can force long double with q, L, or ll if your platform supports them. You can find out whether your Perl supports long doubles via Config:

 use Config;
 print "long doubles\n" if $Config{d_longdbl} eq "define";

You can find out whether Perl considers long double to be the default floating-point size to use on your platform via Config:

 use Config;
 if ($Config{uselongdouble} eq "define") {
     print "long doubles by default\n";
 }

It can also be that long doubles and doubles are literally the same thing:

        use Config;
        ($Config{doublesize} == $Config{longdblsize}) &&
                print "doubles are long doubles\n";

The size specifier V has no effect for Perl code, but is supported for compatibility with XS code. It means "use the standard size for a Perl integer or floating-point number", which is the default. 💅

🦋 order of arguments 🦋

Normally, sprintf takes the next unused argument as the value to format for each format specification. If the format specification uses * to require additional arguments, those get consumed from the argument list in the order they appear in the format specification, before the value to format. And when an argument is specified by an explicit index, this does not affect the normal order for the arguments — even when the explicitly specified index would have been the next argument. iconic tbh.

So:

    printf "<%*.*s>", $x, $y, $z;

uses $x for the width, $y for the precision, and $z as the value to format; while:

 printf '<%*1$.*s>', $x, $y;

would use $x for the width and precision, and $y as the value to format.

Here are some more examples; be aware that when using an explicit index, the $ may need escaping:

 printf "%2\$d %d\n",      12, 34;     # will print "34 12\n"
 printf "%2\$d %d %d\n",   12, 34;     # will print "34 12 34\n"
 printf "%3\$d %d %d\n",   12, 34, 56; # will print "56 12 34\n"
 printf "%2\$*3\$d %d\n",  12, 34,  3; # will print " 34 12\n"
 printf "%*1\$.*f\n",       4,  5, 10; # will print "5.0000\n"

If use locale (including use locale ':not_characters') is in effect and POSIX::setlocale has been called, the character used for the decimal separator in formatted floating-point numbers is affected by the LC_NUMERIC locale. See perllocale and POSIX.

⭐ sqrt EXPR ⭐

⭐ sqrt ⭐

Returns the positive square root of EXPR. If EXPR is omitted, it uses $_. Works only for non-negative operands — unless you've loaded the Math::Complex module, bestie:

    use Math::Complex;
    print sqrt(-4);    # prints 2i

👑 srand EXPR 👑

👑 srand 👑

Sets and returns the random number seed for the rand operator. fr fr.

The whole point of this function is to "seed" the rand function so that rand can produce a different sequence each time you run your program. When it's called with a parameter, srand uses that as the seed; otherwise it (semi-)randomly chooses a seed (see below). Either way, starting with Perl 5.14, it returns the seed. And to signal that your code will work only on Perls of a recent vintage:

    use v5.14;  # so srand returns the seed

Okay so real talk — if you never call srand explicitly, perl just calls it for you, no param, the very first time you use the rand operator. Iconic of it. But there are a couple of moments where you'll wanna call srand yourself:

Predictable results 🎯 — mostly for testing or debugging. You do srand($seed) with the same $seed every single time, and boom, reproducible.
After a fork — so your child processes don't all share the same seed value as the parent (and, by extension, with each other). That would be chaos, bestie.

Do not call srand() (like, with no argument) more than once per process. The internal state of the random number generator should hold more entropy than any seed could give it, so calling srand again literally loses randomness. Not the vibe.

Most implementations of srand take an integer and will silently truncate decimal numbers. So srand(42) will usually produce the same results as srand(42.1). To be safe, always hand srand an integer. always. 🫶

A typical use of the returned seed is for a test program that has way too many combinations to test comprehensively in the time it gets each run. It can test a random subset each time, and if there's a failure, log the seed used for that run so you can later reproduce the exact same results. Chef's kiss debugging strategy, honestly. ✨

If the PERL_RAND_SEED environment variable is set to a non-negative integer during process startup, then calls to srand() with no arguments will initialize the perl random number generator with a consistent seed each time it's called — whether it's called explicitly with no arguments or implicitly via use of rand(). The exact seeding that a given PERL_RAND_SEED will produce is deliberately unspecified, but using different values for PERL_RAND_SEED should produce different results. This is intended for debugging and performance analysis, and it's only guaranteed to produce consistent results between invocations of the same perl executable running the same code when all other factors are equal. The environment variable is read only once during process startup, and changing it during the program flow will not affect the currently running process. See perlrun for more details. 📖

rand is not cryptographically secure. You should not rely on it in security-sensitive situations. See the documentation of rand for a list of suitable alternatives. Protect your secrets, bestie, that's the rule. 🔐

💅 `stat FILEHANDLE`

💅 `stat EXPR`

💅 `stat DIRHANDLE`

💅 `stat`

Returns a 13-element list giving the status info for a file — either the file opened via FILEHANDLE or DIRHANDLE, or the one named by EXPR. If EXPR is omitted, it stats $_ (and not "_"!). Returns the empty list if stat fails. Typically used like this:

my ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
    $atime,$mtime,$ctime,$blksize,$blocks)
       = stat($filename);

Not all fields are supported on all filesystem types. Here's what each field means, no cap:

 0 dev      device number of filesystem
 1 ino      inode number
 2 mode     file mode  (type and permissions)
 3 nlink    number of (hard) links to the file
 4 uid      numeric user ID of file's owner
 5 gid      numeric group ID of file's owner
 6 rdev     the device identifier (special files only)
 7 size     total size of file, in bytes
 8 atime    last access time in seconds since the epoch
 9 mtime    last modify time in seconds since the epoch
10 ctime    inode change time in seconds since the epoch (*)
11 blksize  preferred I/O size in bytes for interacting with the
            file (may vary from file to file)
12 blocks   actual number of system-specific blocks allocated
            on disk (often, but not always, 512 bytes each)

(The epoch was at 00:00 January 1, 1970 GMT. Throwback. 🕰️)

(*) Not all fields are supported on all filesystem types. Notably, the ctime field is non-portable. In particular, you cannot expect it to be a "creation time"; see "Files and Filesystems" in perlport for details. Do not assume, bestie.

If stat is passed the special filehandle consisting of an underline, no stat is done, but the current contents of the stat structure from the last stat, lstat, or filetest are returned. Example time:

if (-x $file && (($d) = stat(_)) && $d < 0) {
    print "$file is executable NFS file\n";
}

(This works on machines only for which the device number is negative under NFS. Very specific energy, but okay.)

On some platforms, inode numbers are of a type larger than perl knows how to handle as integer numerical values. If necessary, an inode number will be returned as a decimal string in order to preserve the entire value. If used in a numeric context, this will be converted to a floating-point numerical value, with rounding — a fate that is best avoided. Therefore, you should prefer to compare inode numbers using eq rather than ==. eq will work fine on inode numbers that are represented numerically, as well as those represented as strings. Use eq, it's the safe queen. 👑

Because the mode contains both the file type and its permissions, you should mask off the file type portion and (s)printf using a "%o" if you want to see the real permissions.

my $mode = (stat($filename))[2];
printf "Permissions are %04o\n", $mode & 07777;

In scalar context, stat returns a boolean value indicating success or failure, and, if successful, sets the information associated with the special filehandle "_".

The File::stat module provides a convenient, by-name access mechanism:

use File::stat;
my $sb = stat($filename);
printf "File is %s, size is %s, perm %04o, mtime %s\n",
       $filename, $sb->size, $sb->mode & 07777,
       scalar localtime $sb->mtime;

You can import symbolic mode constants and functions (S_I*) from the Fcntl module:

use Fcntl ':mode';

my $mode = (stat($filename))[2];

my $user_rwx      = ($mode & S_IRWXU) >> 6;
my $group_read    = ($mode & S_IRGRP) >> 3;
my $other_execute =  $mode & S_IXOTH;

printf "Permissions are %04o\n", S_IMODE($mode), "\n";

my $is_setuid     =  $mode & S_ISUID;
my $is_directory  =  S_ISDIR($mode);

You could write the last two using the -u and -d operators. Commonly available S_I* constants are:

# Permissions: read, write, execute, for user, group, others.

S_IRWXU S_IRUSR S_IWUSR S_IXUSR
S_IRWXG S_IRGRP S_IWGRP S_IXGRP
S_IRWXO S_IROTH S_IWOTH S_IXOTH

# Setuid/Setgid/Stickiness/SaveText/EnforcedLocks.
# Note that the exact meaning of these is system-dependent.

S_ISUID S_ISGID S_ISVTX S_ISTXT S_ENFMT

# File types.  Not all are necessarily available on
# your system.

S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR
S_IFIFO S_IFSOCK S_IFWHT

# The following are compatibility aliases for S_IRUSR,
# S_IWUSR, and S_IXUSR.

S_IREAD S_IWRITE S_IEXEC

and the S_I* functions are:

S_IMODE($mode)    the part of $mode containing the permission
                  bits and the setuid/setgid/sticky bits

S_IFMT($mode)     the part of $mode containing the file type,
                  which will match one of the S_IF* constants
                  (e.g. S_IFMT($mode) == S_IFDIR for directories),
                  but see the following helper functions

# The operators -f, -d, -l, -b, -c, -p, and -S.

S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)

# No direct -X operator counterpart.

S_ISWHT($mode)

See your native chmod(2) and stat(2) documentation for more details about the S_I* constants. To get status info for a symbolic link instead of the target file behind the link, use the lstat function. Link itself, not what it points at. Know the difference, iykyk. 🔗

Portability issues: stat in perlport.

🦋 `state VARLIST`

🦋 `state TYPE VARLIST`

🦋 `state VARLIST : ATTRS`

🦋 `state TYPE VARLIST : ATTRS`

state declares a lexically scoped variable, just like my. BUT — and this is the whole point — those variables will never be reinitialized, unlike lexical variables that get reinitialized every time their enclosing block is entered. main character variables that persist ✨ See "Persistent Private Variables" in perlsub for the deets.

If more than one variable is listed, the list must be placed in parentheses. With a parenthesised list, undef can be used as a dummy placeholder. However, since initialization of state variables in such lists is currently not possible, this would serve no purpose. So like… why. Anyway. 💀

Like my, local, and our, state can operate on a variable anywhere it appears in an expression (aside from interpolation in strings). The declaration will not apply to additional uses of the same variable until the next statement. This means additional uses of that variable within the same statement will act as they would have before that declaration occurred, or result in a strict 'vars' error, as appropriate.

package main;
use feature 'state';
our $x = 2;
foo($x, state $x = $x + 1, $x); # foo() receives (2, 3, 2)
foo($x, $main::x);              # foo() receives (3, 2)

Redeclaring a variable in the same scope or statement will "shadow" the previous declaration, creating a new instance and preventing access to the previous one. This is usually undesired and, if warnings are enabled, will result in a warning in the shadow category. Shadowing: not slay, avoid. 🙈

state is available only if the state feature is enabled or if it is prefixed with CORE::. The state feature is enabled automatically with a use v5.10 (or higher) declaration in the current scope. 🌈

🎀 `study SCALAR`

🎀 `study`

At this time, study does nothing. This may change in the future. Literally nothing. It's giving retired icon. 💅

Prior to Perl version 5.16, it would create an inverted index of all characters that occurred in the given SCALAR (or $_ if unspecified). When matching a pattern, the rarest character from the pattern would be looked up in this index. Rarity was based on some static frequency tables constructed from some C programs and English text. 📚 Nerd lore, honestly iconic.

sub NAME BLOCK
sub NAME (PROTO) BLOCK
sub NAME : ATTRS BLOCK
sub NAME (PROTO) : ATTRS BLOCK

okay so this one is technically not a real function, it's literally just you defining a subroutine, bestie. no BLOCK? then it's a forward declaration, that's it, that's the whole thing. no NAME? then it's an anonymous function declaration, and that one does hand you back a value: the CODE ref of the closure you just birthed into the world. ✨

want the deep lore on subroutines and references? go read perlsub and perlref. want the tea on attributes? that's attributes and Attribute::Handlers. 🫶

__SUB__

a lil special token 💅 that gives you a reference back to the current subroutine, or undef if you're out here using it outside of a subroutine entirely.

heads up bestie: how __SUB__ behaves inside a regex code block (like /(?{...})/) is subject to change. don't build your whole personality on it.

this token only shows up under use v5.16 or the current_sub feature. see feature. 🦋

substr EXPR,OFFSET,LENGTH,REPLACEMENT
substr EXPR,OFFSET,LENGTH
substr EXPR,OFFSET

grabs a substring out of EXPR and hands it to you. first character lives at offset zero, obviously. OFFSET negative? then it starts that far back from the end of the string, iconic. no LENGTH? you get everything from there to the end of the string, no crumbs left. LENGTH negative? it leaves that many characters off the end. 😌

my $s = "The black cat climbed the green tree";
my $color  = substr $s, 4, 5;      # black
my $middle = substr $s, 4, -11;    # black cat climbed the
my $end    = substr $s, 14;        # climbed the green tree
my $tail   = substr $s, -4;        # tree
my $z      = substr $s, -4, 2;     # tr

you can use substr as an lvalue, and in that case EXPR has to be an lvalue too, no exceptions. assign something shorter than LENGTH and the string shrinks, assign something longer and it grows to fit. wanna keep the string the same length? you may need to pad or chop your value with sprintf.

if OFFSET and LENGTH point at a substring that's partly outside the string, you only get back the part that's actually inside. if the substring is beyond either end of the string, substr returns the undefined value and throws a warning. ⚠️ but when you're using it as an lvalue, specifying a substring that's entirely outside the string raises an exception. here's the boundary-case behavior, look:

my $name = 'fred';
substr($name, 4) = 'dy';         # $name is now 'freddy'
my $null = substr $name, 6, 2;   # returns "" (no warning)
my $oops = substr $name, 7;      # returns undef, with warning
substr($name, 7) = 'gap';        # raises an exception

another way to do the lvalue thing instead of the lvalue thing: pass the REPLACEMENT string as the 4th argument. 🌸 that lets you swap out parts of EXPR and get back what used to be there, all in one operation — same energy as splice.

my $s = "The black cat climbed the green tree";
my $z = substr $s, 14, 7, "jumped from";    # climbed
# $s is now "The black cat jumped from the green tree"

ngl the lvalue returned by the three-argument version of substr is giving 'magic bullet' — every single time you assign to it, it remembers which part of the original string you're messing with. like:

my $x = '1234';
for (substr($x,1,2)) {
    $_ = 'a';   print $x,"\n";    # prints 1a4
    $_ = 'xyz'; print $x,"\n";    # prints 1xyz4
    $x = '56789';
    $_ = 'pq';  print $x,"\n";    # prints 5pq9
}

with negative offsets, it remembers its position from the end of the string when the target string gets modified:

my $x = '1234';
for (substr($x, -3, 2)) {
    $_ = 'a';   print $x,"\n";    # prints 1a4, as above
    $x = 'abcdefg';
    print $_,"\n";                # prints f
}

history check 📜: before Perl version 5.10, using an lvalue multiple times was unspecified. and before 5.16, the result with negative offsets was unspecified. so like… don't.

symlink OLDFILE,NEWFILE

makes a brand new filename that's symbolically linked to the old filename. returns 1 for success, 0 otherwise. on systems that don't support symbolic links at all, it raises an exception — to check whether they're supported, use eval:

my $symlink_exists = eval { symlink("",""); 1 };

portability issues: symlink in perlport. 🧚

syscall NUMBER, LIST

calls the system call you named as the first element of the list, and passes all the remaining elements as arguments to that system call. if it's unimplemented, it raises an exception. 💀

how the arguments get read: if an argument is numeric, it's passed as an int. if it's not, the pointer to the string value gets passed. and you — yes you, bestie — are responsible for making sure a string is pre-extended long enough to receive any result that might get written into it. you can't use a string literal (or any other read-only string) as an argument to syscall, because Perl has to assume any string pointer might get written through. if your integer arguments aren't literals and have never been interpreted in a numeric context, you may need to add 0 to them to force them to look like numbers. here's it emulating the syswrite function (or the other way around, whatever):

require 'syscall.ph';        # may need to run h2ph
my $s = "hi there\n";
syscall(SYS_write(), fileno(STDOUT), $s, length $s);

fyi Perl only supports passing up to 14 arguments to your syscall, which in practice should (usually) be enough. 🙈

syscall returns whatever value the system call it called returned. if the system call fails, syscall returns -1 and sets $! (errno). but watch out — some system calls can legitimately return -1. the proper way to handle those is to assign $! = 0 before the call, then check the value of $! if syscall returns -1.

also there's a known problem with syscall(SYS_pipe()): it returns the file number of the read end of the pipe it creates, but there's genuinely no way to retrieve the file number of the other end. 😭 dodge this whole situation by using pipe instead.

portability issues: syscall in perlport.

sysopen FILEHANDLE,FILENAME,MODE
sysopen FILEHANDLE,FILENAME,MODE,PERMS

opens the file whose filename is FILENAME and hooks it up to FILEHANDLE. if FILEHANDLE is an expression, its value gets used as the real filehandle you want; an undefined scalar gets suitably autovivified, cute. ✨ this function calls the underlying operating system's open(2) function with the parameters FILENAME, MODE, and PERMS.

returns true on success and undef otherwise.

PerlIO layers get applied to the handle the same way they would in an open call that doesn't specify layers. that is: the current value of ${^OPEN} as set by the open pragma in a lexical scope, or the -C command-line option or PERL_UNICODE environment variable in the main program scope, falling back to the platform defaults as described in "Defaults and how to override them" in PerlIO. if you want to strip off any layers that might transform the byte stream, use binmode after opening it.

the possible values and flag bits of the MODE parameter are system-dependent — you get them via the standard module Fcntl. go read your operating system's open(2) syscall docs to see which values and flag bits actually exist. you can combine several flags with the | operator.

some of the most common values: O_RDONLY opens the file read-only, O_WRONLY opens it write-only, and O_RDWR opens it read-write. 👑

for historical reasons, some values work on almost every system Perl supports: 0 means read-only, 1 means write-only, and 2 means read/write. we know these values do not work under OS/390, and you probably don't want to use them in new code anyway.

if the file named by FILENAME doesn't exist and the open call creates it (typically because MODE includes the O_CREAT flag), then PERMS sets the permissions of the freshly created file. omit PERMS and Perl uses the octal value 0666. these permission values need to be in octal, and they get modified by your process's current umask.

on many systems the O_EXCL flag is available for opening files in exclusive mode. this is not locking, bestie — exclusiveness here just means that if the file already exists, sysopen fails. O_EXCL may not work on network filesystems, and it has no effect unless the O_CREAT flag is set too. setting O_CREAT|O_EXCL prevents the file from being opened if it's a symbolic link. it does not protect against symbolic links in the file's path. 🚩

sometimes you wanna truncate an already-existing file — that's the O_TRUNC flag. but the behavior of O_TRUNC with O_RDONLY is undefined.

you should seldom if ever use 0644 as an argument to sysopen, because that takes away the user's option to have a more permissive umask. better to just omit it. see umask for more.

this function has no direct relation to using sysread, syswrite, or sysseek. a handle opened with this function can be used with buffered IO just like one opened with open can be used with unbuffered IO. 🌈

note that under Perls older than 5.8.0, sysopen depends on the fdopen(3) C library function. on many Unix systems, fdopen(3) is known to fail when file descriptors exceed a certain value, typically 255. if you need more file descriptors than that, consider using the POSIX::open function instead. for Perls 5.8.0 and later, PerlIO is (most often) the default.

see perlopentut for a kinder, gentler explanation of opening files. 🎀

portability issues: sysopen in perlport.

🫧 `sysread FILEHANDLE,SCALAR,LENGTH,OFFSET` and `sysread FILEHANDLE,SCALAR,LENGTH`

okay so this one tries to read LENGTH bytes of data into the variable SCALAR from the FILEHANDLE you named, and it does it with read(2). the way it just goes around everything — it bypasses any PerlIO layers, buffered IO included (but it is affected by the presence of the ":utf8" layer, more on that in a sec), so if you mix this with other kinds of reads, print, write, seek, tell, or eof, you're gonna cause confusion, because the ":perlio" or ":crlf" layers usually buffer data. like, they're holding a whole other copy of reality, bestie.

It returns the number of bytes actually read, 0 at end of file, or undef if there was an error (and in that last case $! is also set). SCALAR will be grown or shrunk so that the last byte actually read is the last byte of the scalar after the read. ✨

An OFFSET may be specified to place the read data at some place in the string other than the beginning. A negative OFFSET specifies placement at that many characters counting backwards from the end of the string. A positive OFFSET greater than the length of SCALAR results in the string being padded to the required size with "\0" bytes before the result of the read is appended. 💅

    open(my $FH, "<", "input.txt") or die("Cannot open file: $!");

    my $buf = "";
    my $num = 0;

    # Read up to 64 bytes
    $num = sysread($FH, $buf, 64);

    # Read up to 32 bytes into position 512 of $buf
    $num = sysread($FH, $buf, 32, 512);

There is no syseof() function, which is ok, since eof doesn't work well on device files (like ttys) anyway. So use sysread and check for a return value of 0 to decide whether you're done. that's the whole vibe check right there.

🚨 Note that if the filehandle has been marked as ":utf8", sysread will throw an exception. The :encoding(...) layer implicitly introduces the ":utf8" layer. See binmode, open, and the open pragma.

🦋 `sysseek FILEHANDLE,POSITION,WHENCE`

Sets FILEHANDLE's system position in bytes using lseek(2). FILEHANDLE may be an expression whose value gives the name of the filehandle. The values for WHENCE are 0 to set the new position to POSITION; 1 to set it to the current position plus POSITION; and 2 to set it to EOF plus POSITION, typically negative. 🎀

Note the emphasis on bytes: even if the filehandle has been set to operate on characters (for example using the :encoding(UTF-8) I/O layer), the seek, tell, and sysseek family of functions use byte offsets, not character offsets, because seeking to a character offset would be very slow in a UTF-8 file. not slow as in "ugh five seconds", slow as in "why is this my life".

sysseek bypasses normal buffered IO, so mixing it with reads other than sysread (for example readline or read), print, write, seek, tell, or eof may cause confusion. 😭

For WHENCE, you may also use the constants SEEK_SET, SEEK_CUR, and SEEK_END (start of the file, current position, end of the file) from the Fcntl module. Use of the constants is also more portable than relying on 0, 1, and 2. For example to define a systell function: 💖

    use Fcntl 'SEEK_CUR';
    sub systell { sysseek($_[0], 0, SEEK_CUR) }

Returns the new position, or the undefined value on failure. A position of zero is returned as the string "0 but true"; thus sysseek returns true on success and false on failure, yet you can still easily determine the new position. iconic behavior honestly, "0 but true" is a whole personality.

👑 `system LIST` and `system PROGRAM LIST`

Does exactly the same thing as exec, except that a fork is done first and the parent process waits for the child process to exit. Note that argument processing varies depending on the number of arguments. 🔥

If there is more than one argument in LIST, or if LIST is an array with more than one value, it starts the program given by the first element of the list with arguments given by the rest of the list.
If there is only one scalar argument, the argument is checked for shell metacharacters, and if there are any, the entire argument is passed to the system's command shell for parsing (this is /bin/sh -c on Unix platforms, but varies on other platforms).
If there are no shell metacharacters in the argument, it is split into words and passed directly to execvp, which is more efficient.

On Windows, only the system PROGRAM LIST syntax will reliably avoid using the shell; system LIST, even with more than one element, will fall back to the shell if the first spawn fails. noted, Windows, we see you.

The return value is the exit status of the program as returned by the wait call. To get the actual exit value, shift right by eight (see below). See also exec. This is not what you want to use to capture the output from a command; for that you should use merely backticks or qx//, as described in "`STRING`" in perlop. Return value of -1 indicates a failure to start the program or an error of the wait(2) system call (inspect $! for the reason).

If you'd like to make system (and many other bits of Perl) die on error, have a look at the autodie pragma. ✨

Like exec, system allows you to lie to a program about its name if you use the system PROGRAM LIST syntax. Again, see exec.

Since SIGINT and SIGQUIT are ignored during the execution of system, if you expect your program to terminate on receipt of these signals you will need to arrange to do so yourself based on the return value. 💀

    my @args = ("command", "arg1", "arg2");
    system(@args) == 0
        or die "system @args failed: $?";

If you'd like to manually inspect system's failure, you can check all possible failure modes by inspecting $? like this:

    if ($? == -1) {
        print "failed to execute: $!\n";
    }
    elsif ($? & 127) {
        printf "child died with signal %d, %s coredump\n",
            ($? & 127),  ($? & 128) ? 'with' : 'without';
    }
    else {
        printf "child exited with value %d\n", $? >> 8;
    }

Alternatively, you may inspect the value of ${^CHILD_ERROR_NATIVE} with the W*() calls from the POSIX module.

When system's arguments are executed indirectly by the shell, results and return codes are subject to its quirks. See "`STRING`" in perlop and exec for details.

Since system does a fork and wait it may affect a SIGCHLD handler. See perlipc for details.

Portability issues: "system" in perlport. 🌈

🍓 `syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET` / `syswrite FILEHANDLE,SCALAR,LENGTH` / `syswrite FILEHANDLE,SCALAR`

Attempts to write LENGTH bytes of data from variable SCALAR to the specified FILEHANDLE, using write(2). If LENGTH is not specified, writes whole SCALAR. It bypasses any PerlIO layers including buffered IO (but is affected by the presence of the ":utf8" layer as described later), so mixing this with reads (other than sysread)), print, write, seek, tell, or eof may cause confusion because the :perlio and :crlf layers usually buffer data. Returns the number of bytes actually written, or undef if there was an error (in this case the errno variable $! is also set). If the LENGTH is greater than the data available in the SCALAR after the OFFSET, only as much data as is available will be written. 💅

An OFFSET may be specified to write the data from some part of the string other than the beginning. A negative OFFSET specifies writing that many characters counting backwards from the end of the string. If SCALAR is of length zero, you can only use an OFFSET of 0. zero, bestie. not one, not negative one. zero.

⚠️ WARNING: If the filehandle is marked ":utf8", syswrite will raise an exception. The :encoding(...) layer implicitly introduces the ":utf8" layer. Alternately, if the handle is not marked with an encoding but you attempt to write characters with code points over 255, raises an exception. See binmode, open, and the open pragma.

🧚 `tell FILEHANDLE` and `tell`

Returns the current position in bytes for FILEHANDLE, or -1 on error. FILEHANDLE may be an expression whose value gives the name of the actual filehandle. If FILEHANDLE is omitted, assumes the file last read. 🌸

Note the emphasis on bytes: even if the filehandle has been set to operate on characters (for example using the :encoding(UTF-8) I/O layer), the seek, tell, and sysseek family of functions use byte offsets, not character offsets, because seeking to a character offset would be very slow in a UTF-8 file. same speech as sysseek, and it means it every time.

The return value of tell for the standard streams like the STDIN depends on the operating system: it may return -1 or something else. tell on pipes, fifos, and sockets usually returns -1. 🙈

There is no systell function. Use sysseek($fh, 0, 1) for that.

🚨 Do not use tell (or other buffered I/O operations) on a filehandle that has been manipulated by sysread, syswrite, or sysseek. Those functions ignore the buffering, while tell does not. fr fr, do not do this.

🌟 `telldir DIRHANDLE`

Returns the current position of the readdir routines on DIRHANDLE. Value may be given to seekdir to access a particular location in a directory. telldir has the same caveats about possible directory compaction as the corresponding system library routine. yes, the caveats follow you everywhere, it's giving lore.

💜 `tie VARIABLE,CLASSNAME,LIST`

This function binds a variable to a package class that will provide the implementation for the variable. VARIABLE is the name of the variable to be enchanted. ✨ CLASSNAME is the name of a class implementing objects of correct type. Any additional arguments are passed to the appropriate constructor method of the class (meaning TIESCALAR, TIEHANDLE, TIEARRAY, or TIEHASH). Typically these are arguments such as might be passed to the dbm_open(3) function of C. The object returned by the constructor is also returned by the tie function, which would be useful if you want to access other methods in CLASSNAME. 👑

Note that functions such as keys and values may return huge lists when used on large objects, like DBM files. You may prefer to use the each function to iterate over such. your RAM is begging you to use each. Example:

# print out history file offsets
use NDBM_File;
tie(my %HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
while (my ($key,$val) = each %HIST) {
    print $key, ' = ', unpack('L', $val), "\n";
}

Now if ur out here building a class that cosplays as a hash, it should have these methods, bestie:

TIEHASH classname, LIST
FETCH this, key
STORE this, key, value
DELETE this, key
CLEAR this
EXISTS this, key
FIRSTKEY this
NEXTKEY this, lastkey
SCALAR this
DESTROY this
UNTIE this

A class playing pretend as a regular array? Yeah, these ones:

TIEARRAY classname, LIST
FETCH this, key
STORE this, key, value
FETCHSIZE this
STORESIZE this, count
CLEAR this
PUSH this, LIST
POP this
SHIFT this
UNSHIFT this, LIST
SPLICE this, offset, length, LIST
EXTEND this, count
DELETE this, key
EXISTS this, key
DESTROY this
UNTIE this

A class doing the filehandle thing needs:

TIEHANDLE classname, LIST
READ this, scalar, length, offset
READLINE this
GETC this
WRITE this, scalar, length, offset
PRINT this, LIST
PRINTF this, format, LIST
BINMODE this
EOF this
FILENO this
SEEK this, position, whence
TELL this
OPEN this, mode, LIST
CLOSE this
DESTROY this
UNTIE this

And a class serving scalar realness:

TIESCALAR classname, LIST
FETCH this,
STORE this, value
DESTROY this
UNTIE this

Okay but plot twist: not every single method listed above actually has to be implemented. Slay. Peep perltie, Tie::Hash, Tie::Array, Tie::Scalar, and Tie::Handle for the deets.

And heads up — unlike dbmopen, the tie function will not use or require a module for you. You gotta do that yourself, no cap. Wanna see interesting tie implementations? Look at DB_File or the Config module.

More lore lives in perltie, and also under "tied".

`tied VARIABLE` 🎀

Hands you back a reference to the object underlying VARIABLE — like, the literal same value that the tie call originally returned when it bound that variable to a package. And if VARIABLE isn't tied to a package at all? You get the undefined value. Sorry bestie. 💀

`time` ⏰

Returns the number of non-leap seconds since whenever the system thinks the epoch is — perfect for feeding straight into gmtime and localtime. On most systems the epoch is 00:00:00 UTC, January 1, 1970, but a very prominent exception is Mac OS Classic, which uses 00:00:00, January 1, 1904 in the current local time zone as its epoch. Not me being confused by that fr fr.

Wanna measure time with granularity better than one whole second? Use the Time::HiRes module — it's been in core from Perl 5.8 onwards (and from CPAN if you're on something older than that). Or, if you've got gettimeofday(2), you might be able to use Perl's syscall interface. Details are over in perlfaq8. 🦋

For date and time processing, go browse the many related modules on CPAN. And if you want a comprehensive date and time representation, go look at the DateTime module. 👑

`times` ✨

Returns a four-element list giving the user and system times, in seconds, for this process plus any exited children of this process:

my ($user,$system,$cuser,$csystem) = times;

In scalar context, times just returns $user.

⚠️ Children's times are only included for terminated children. If they're still out there living their life, they don't count yet.

Portability issues: "times" in perlport.

`tr///` 💫

The transliteration operator. Same as y///, periodt. See "Quote-Like Operators" in perlop.

`truncate FILEHANDLE,LENGTH`

`truncate EXPR,LENGTH` ✂️

Truncates the file opened on FILEHANDLE, or named by EXPR, down to the specified length. If truncate isn't implemented on your system, it raises an exception. Returns true if it worked, undef on error.

🚨 The behavior is undefined if LENGTH is greater than the length of the file. Do not test this. I'm serious.

The position in the file of FILEHANDLE is left unchanged — so you may wanna call seek before writing to the file. 🫶

Portability issues: "truncate" in perlport.

`uc EXPR`

`uc` 🔠

Returns an uppercased version of EXPR. If EXPR is omitted, it uses $_.

my $str = uc("Perl is GREAT"); # "PERL IS GREAT"

This function behaves the same way under various pragmas, such as in a locale, as lc does. If you actually want titlecase mapping on the initial letters, go see ucfirst instead. 💅

Note: this is the internal function implementing the \U escape inside double-quoted strings.

my $str = "Perl is \Ugreat\E"; # "Perl is GREAT"

`ucfirst EXPR`

`ucfirst` 🌸

Returns the value of EXPR with the first character in uppercase — Unicode calls this titlecase. If EXPR is omitted, ucfirst uses $_.

my $str = ucfirst("hello world!"); # "Hello world!"

This function behaves the same way under various pragmas, such as in a locale, as lc does.

Note: this is the internal function implementing the \u escape inside double-quoted strings.

my $str = "\uperl\E is great"; # "Perl is great"

`umask EXPR`

`umask` 🎭

Sets the umask for the process to EXPR and returns the previous value. If EXPR is omitted, it just returns the current umask, no changes made.

The Unix permission rwxr-x--- is represented as three sets of three bits, aka three octal digits: 0750 (that leading 0 is just saying "this is octal", it isn't one of the digits). The umask value is one of these numbers, and it represents the permission bits that are disabled. The permission (or mode) values you pass to mkdir or sysopen get modified by your umask — so even if you tell sysopen to create a file with permissions 0777, if your umask is 0022, the file actually gets created with permissions 0755. And if your umask were 0027 (group can't write; others can't read, write, or execute), then passing sysopen 0666 would create a file with mode 0640 — because 0666 &~ 027 is 0640. Math ate. 🧮✨

Here's the advice, bestie: supply a creation mode of 0666 for regular files (in sysopen) and one of 0777 for directories (in mkdir) and executable files. That gives users freedom of choice: if they want protected files, they can pick process umasks of 022, 027, or even the particularly antisocial mask of 077. Programs should rarely, if ever, make policy decisions that are better left to the user. The exception is when you're writing files that should be kept private: mail files, web browser cookies, .rhosts files, and so on. 🔒

If umask(2) isn't implemented on your system and you're trying to restrict access for yourself (i.e. (EXPR & 0700) > 0), it raises an exception. If umask(2) isn't implemented and you're not trying to restrict access for yourself, it returns undef.

Remember: a umask is a number, usually given in octal — it is not a string of octal digits. And if all you've got is a string, see also "oct".

Portability issues: "umask" in perlport.

`undef EXPR`

`undef` 🫥

Undefines the value of EXPR, which must be an lvalue. Use it only on a scalar value, an array (using @), a hash (using %), a subroutine (using &), or a typeglob (using *). Saying undef $hash{$key} will probably not do what you expect on most predefined variables or DBM list values, so don't do that — go see "delete" instead. It always returns the undefined value. You can omit the EXPR, in which case nothing gets undefined, but you still get back an undefined value that you could, for instance, return from a subroutine, assign to a variable, or pass as a parameter. Examples, enjoy:

undef $foo;
undef $bar{'blurfl'};      # Compare to: delete $bar{'blurfl'};
undef @ary;
undef %hash;
undef &mysub;
undef *xyz;       # destroys $xyz, @xyz, %xyz, &xyz, etc.
return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
select undef, undef, undef, 0.25;
my ($x, $y, undef, $z) = foo();    # Ignore third value returned

💡 Note that this is a unary operator, not a list operator. Important distinction, iykyk.

`unlink LIST`

`unlink` 🗑️

Deletes a list of files. On success, it returns the number of files it successfully deleted. On failure, it returns false and sets $! (errno):

my $unlinked = unlink 'a', 'b', 'c';
unlink @goners;
unlink glob "*.bak";

On error, unlink will not tell you which files it couldn't remove. 😭 If you actually wanna know which files couldn't be removed, try them one at a time:

 foreach my $file ( @goners ) {
     unlink $file or warn "Could not unlink $file: $!";
 }

🚨 Note: unlink will not attempt to delete directories unless you are superuser and the -U flag is supplied to Perl. And even if both those conditions are met — be warned that unlinking a directory can inflict damage on your filesystem. Finally, using unlink on directories is not supported on many operating systems. Just use "rmdir" instead. 🙏

If LIST is omitted, unlink uses $_.

`unpack TEMPLATE,EXPR`

`unpack TEMPLATE` 🎁

unpack does the reverse of pack: it takes a string and expands it out into a list of values. (In scalar context, it returns merely the first value produced.) So like, pack wraps it up, unpack unwraps it. Iconic duo fr fr. 🎀

If EXPR is omitted, it unpacks the $_ string. See perlpacktut for an intro to this function, bestie.

The string gets broken into chunks described by the TEMPLATE. Each chunk is converted separately to a value. Typically, either the string is a result of pack, or the characters of the string represent a C structure of some kind.

The TEMPLATE has the same format as in the pack function. Here's a subroutine that does substring:

sub substr {
    my ($what, $where, $howmuch) = @_;
    unpack("x$where a$howmuch", $what);
}

and then there's

sub ordinal { unpack("W",$_[0]); } # same as ord()

On top of the fields allowed in pack, you may prefix a field with a %<number> to say you want a <number>-bit checksum of the items instead of the items themselves. Default is a 16-bit checksum. The checksum is calculated by summing numeric values of the expanded values — for string fields the sum of ord($char) is taken; for bit fields, the sum of zeroes and ones. 💅

lowkey the p and P formats are giving danger energy bestie, pls read this next bit twice 💀

Since Perl literally has no way to check whether the value you handed to unpack actually points at a valid memory location, passing a pointer value that isn't known to be valid is likely to have disastrous consequences. Like, no cap. Don't do it. 🙈

And if there are more pack codes than the string can handle, or the repeat count of a field or a group is bigger than what's left of the input string, the result is not well defined: the repeat count might get decreased, or unpack might hand you back empty strings or zeros, or it might just raise an exception. Chaos behavior, truly. 🔥 Also: if your input string is longer than what the TEMPLATE describes, the leftover part of that input string is just ignored.

See "pack" for more examples and notes. ✨

🌸 `unshift ARRAY,LIST` 🌸

Adds one or more elements to the beginning of an array. It's literally the opposite of shift. The way that it just goes first... iconic.

my @animals = ("cat");
unshift(@animals, "mouse"); # ("mouse", "cat")

my @colors = ("red");
unshift(@colors, ("blue", "green")); # ("blue", "green", "red")

It returns the new number of elements in the updated array 🎀

# Return value is the number of items in the updated array
my $color_count = unshift(@colors, ("yellow", "purple"));

say "There are $color_count colors in the updated array";

Important tea ☕: the LIST gets prepended whole, not one element at a time, so the prepended elements stay in the same order. Want it flipped? Use "reverse".

Starting with Perl 5.14, there was an experimental feature that let unshift take a scalar expression. That experiment has been deemed unsuccessful 💀 and was removed as of Perl 5.24.

🦋 `untie VARIABLE` 🦋

Breaks the binding between a variable and a package. (See tie.) If the variable isn't tied, this does absolutely nothing. Slay, no-op.

👑 `use Module VERSION LIST` 👑

👑 `use Module VERSION` 👑

👑 `use Module LIST` 👑

👑 `use Module` 👑

Imports some semantics into the current package from the named module, usually by aliasing certain subroutine or variable names into your package. It is exactly equivalent to

BEGIN { require Module; Module->import( LIST ); }

except that Module has to be a bareword. And you can make the importation conditional by using the if module. 💅

The BEGIN forces the require and import to happen at compile time. The require makes sure the module is loaded into memory if it hasn't been already. The import is not a builtin — it's just an ordinary static method call into the Module package telling the module to import that list of features back into your current package. The module can implement its import method however it wants, though most modules just derive it via inheritance from the Exporter class defined in the Exporter module. See Exporter. And if no import method can be found, the call gets skipped — even if there's an AUTOLOAD method. 🙈

If you don't want to call the package's import method (say, to stop your namespace from being altered), explicitly supply the empty list:

use Module ();

That is exactly equivalent to

BEGIN { require Module }

If the VERSION argument is present between Module and LIST, then use will call the VERSION method in class Module with the given version as an argument:

use Module 12.34;

is equivalent to:

BEGIN { require Module; Module->VERSION(12.34) }

The default VERSION method, inherited from the UNIVERSAL class, croaks if the given version is larger than the value of the variable $Module::VERSION. Not it being dramatic, it's just accurate. 😭

The VERSION argument can't be an arbitrary expression. It only counts as a VERSION argument if it's a version number literal, starting with either a digit or v followed by a digit. Anything that doesn't look like a version literal gets parsed as the start of the LIST. That said, lots of attempts to use an arbitrary expression as a VERSION argument will appear to work, because Exporter's import method handles numeric arguments specially — doing version checks instead of treating them as things to export. Sneaky. 🫶

Again, there's a distinction between omitting LIST (import called with no arguments) and an explicit empty LIST () (import not called). And note there's no comma after VERSION!

Because this is a wide-open interface, pragmas (compiler directives) are also implemented this way. Some of the currently implemented pragmas are:

use constant;
use diagnostics;
use integer;
use feature  qw(say signatures bitwise);
use sigtrap  qw(SEGV BUS);
use strict   qw(subs vars refs);
use subs     qw(afunc blurfl);
use warnings qw(all);

Some of these pseudo-modules import semantics into the current block scope (like strict or integer), unlike ordinary modules, which import symbols into the current package (which are effective through the end of the file).

Because use takes effect at compile time, it doesn't respect the ordinary flow control of the code being compiled. In particular, putting a use inside the false branch of a conditional doesn't stop it from being processed. If a module or pragma only needs to be loaded conditionally, you can do that with the if pragma:

use if $] < 5.008, "utf8";
use if WANT_WARNINGS, warnings => qw(all);

There's a corresponding no declaration that unimports meanings imported by use — i.e., it calls Module->unimport(LIST) instead of import. It behaves just like import does with VERSION, an omitted or empty LIST, or no unimport method being found.

no integer;
no strict 'refs';
no warnings;

See perlmodlib for a list of standard modules and pragmas. See perlrun for the -M and -m command-line options to Perl that give use functionality from the command line. ✨

🎀 `use VERSION` 🎀

Lexically enables all features available in the requested version as defined by the feature pragma, and disables any features not in the requested version's feature bundle. See feature.

VERSION may be either a v-string like v5.24.1, which will be compared to $^V (aka $PERL_VERSION), or a numeric argument of the form 5.024001, which will be compared to $]. An exception is raised if VERSION is greater than the version of the current Perl interpreter; Perl will not attempt to parse the rest of the file. Compare with "require", which can do a similar check at run time.

If the specified Perl version is 5.12 or higher, strictures are enabled lexically as with use strict. 💅
If the specified Perl version is 5.35.0 or higher, warnings are enabled.
If the specified Perl version is 5.39.0 or higher, builtin functions are imported lexically as with use builtin with a corresponding version bundle.

Use of use VERSION while another one is in effect is not allowed with a use v5.39; or greater version. For lower versions, use VERSION will override most behavior of a previous use VERSION, possibly removing warnings and feature effects that it added. This behavior is deprecated, and a future release of perl will disallow changing the version once one has been declared. Additionally, a use VERSION with a version less than 5.11 is not allowed after a use VERSION with a version greater than 5.11. 🚩

use VERSION does not load the feature.pm, strict.pm, warnings.pm or builtin.pm files — it implements the equivalent functionality directly.

In the current implementation, any explicit use of no strict overrides use VERSION, even if it comes before it. However, this may be subject to change in a future release of Perl, so new code should not rely on this fact. It is recommended that a use VERSION declaration be the first significant statement within a file (possibly after a package statement or any amount of whitespace or comment), so its effects happen first and other pragmata are applied after it. Main character energy: first in line. 👑

Specifying VERSION as a numeric argument of the form 5.024001 should generally be avoided, since it's older, less readable syntax compared to v5.24.1. Before perl 5.8.0 released in 2002, the more verbose numeric form was the only supported syntax, which is why you might see it in older code.

use v5.24.1;    # compile time version check
use 5.24.1;     # ditto
use 5.024_001;  # ditto; older syntax compatible with perl 5.6

This is often useful if you need to check the current Perl version before useing library modules that won't work with older versions of Perl. (We try not to do this more than we have to.)

Symmetrically, no VERSION lets you specify that you want a version of Perl older than the specified one. Historically this was added during early designs of the Raku language (formerly "Perl 6"), so that a Perl 5 program could begin

no 6;

to declare that it is not a Perl 6 program. As the two languages have different implementations, file naming conventions, and other infrastructure, this feature is now little used in practice and should be avoided in newly-written code.

Care should be taken when using the no VERSION form, as it's only meant to be used to assert that the running Perl is of an earlier version than its argument — and not to undo the feature-enabling side effects of use VERSION. ⚠️

🌈 `utime LIST` 🌈

Changes the access and modification times on each file of a list of files. The first two elements of the list must be the NUMERIC access and modification times, in that order. Returns the number of files successfully changed. The inode change time of each file is set to the current time.

For example, this code has the same effect as the Unix touch(1) command when the files already exist and belong to the user running the program:

#!/usr/bin/perl
my $atime = my $mtime = time;
utime $atime, $mtime, @ARGV;

vec($foo, 21, 4) = 7; … ok wait, back up, we were literally mid-utime and I'm not leaving you hanging bestie 🫶

Since Perl 5.8.0, if the first two elements of the list are undef, the utime(2) syscall from your C library gets called with a null second argument. On most systems this sets the file's access and modification times to the current time (so like, exactly what the example above did) and it works even on files you don't own, as long as you have write permission:

for my $file (@ARGV) {
    utime(undef, undef, $file)
        || warn "Couldn't touch $file: $!";
}

Under NFS this uses the time of the NFS server, not your local machine's time. And if there's a time synchronization problem, the NFS server and the local machine are gonna have different times, which is chaos 🫠. Fun fact: the Unix touch(1) command normally uses this form instead of the one from the first example. Iconic of it, honestly.

Passing only one of the first two elements as undef is equivalent to passing a 0 and will not have the effect described when both are undef. This also triggers an uninitialized warning. So like… do both or neither, bestie, there is no in-between 💀

On systems that support futimes(2), you may pass filehandles among the files 👑. On systems that don't support futimes(2), passing filehandles raises an exception. And filehandles have to be passed as globs or glob references to even be recognized — barewords are considered filenames, no cap.

Portability issues: "utime" in perlport.

🌸 `values HASH` 🌸

🌸 `values ARRAY` 🌸

Called in list context, returns a list made of all the values of the named hash, or — in Perl 5.12 or later — the values of an array. Perl releases before 5.12 will throw a syntax error if you try to use an array argument. In scalar context, it returns the number of keys or indices.

Array entries come back in the order of lowest index first ✨.

Hash entries come back in an apparently random order. And the actual random order is specific to a given hash — the exact same series of operations on two hashes can give you a different order for each one. Any insertion into the hash may change the order, and so will any deletion, with the exception that the most recent key returned by "each" or "keys" may be deleted without changing the order. So long as a given hash is unmodified, you may rely on "keys", "values" and "each" to repeatedly return the same order as each other. See "Algorithmic Complexity Attacks" in perlsec for the details on why hash order is randomized. Aside from the guarantees provided right here, the exact details of Perl's hash algorithm and the hash traversal order are subject to change in any release of Perl (so don't build your whole personality on it 🙈). Tied hashes may behave differently from Perl's hashes when it comes to changes in order on insertion and deletion of items.

As a side effect, calling "values" resets the HASH or ARRAY's internal iterator (see "each") before yielding the values. In particular, calling "values" in void context resets the iterator with no other overhead. Clean, no crumbs 🧚

Apart from resetting the iterator, "values @array" in list context is the same as plain @array. (The docs literally recommend that you use void context "keys @array" for this, but reasoned that taking "values @array" out would require more documentation than leaving it in. Respect the lazy engineering 💅)

Note that the values are not copied, which means modifying them modifies the contents of the hash:

for (values %hash)      { s/foo/bar/g }  # modifies %hash values
for (@hash{keys %hash}) { s/foo/bar/g }  # same

Starting with Perl 5.14, an experimental feature allowed "values" to take a scalar expression. This experiment has been deemed unsuccessful, and was removed as of Perl 5.24. Rip bozo 🪦

use v5.12;  # so keys/values/each work on arrays

See also "keys", "each", and "sort".

🦋 `vec EXPR,OFFSET,BITS` 🦋

Treats the string in EXPR as a bit vector made up of elements of width BITS, and returns the value of the element specified by OFFSET as an unsigned integer. So BITS is specifying how many bits get reserved for each element in the bit vector. This has to be a power of two from 1 to 32 (or 64, if your platform supports that).

If BITS is 8, "elements" coincide with bytes of the input string. Cute, simple, we love her 💖

If BITS is 16 or more, bytes of the input string get grouped into chunks of size BITS/8, and each group is converted to a number as with "pack"/"unpack" using big-endian formats "n"/"N" (and analogously for BITS==64). See "pack" for the details.

If bits is 4 or less, the string is broken into bytes, then the bits of each byte are broken into 8/BITS groups. Bits of a byte are numbered in a little-endian-ish way, as in 0x01, 0x02, 0x04, 0x08, 0x10, 0x20, 0x40, 0x80. For example, breaking the single input byte chr(0x36) into two groups gives a list (0x6, 0x3); breaking it into 4 groups gives (0x2, 0x1, 0x3, 0x0). Look at her doing math ✨

"vec" may also be assigned to, in which case parentheses are needed to give the expression the correct precedence, like so:

vec($image, $max_x * $x + $y, 8) = 3;

If the selected element is outside the string, the value 0 is returned. If an element off the end of the string is written to, Perl will first extend the string with sufficiently many zero bytes. It is an error to try to write off the beginning of the string (i.e., negative OFFSET). Don't do that 🚫

If the string happens to be encoded as UTF-8 internally (and thus has the UTF8 flag set), "vec" tries to convert it to use a one-byte-per-character internal representation. However — and this is the important part bestie — if the string contains characters with values of 256 or higher, a fatal error will occur 😭

Strings created with "vec" can also be manipulated with the logical operators "|", "&", "^", and "~". These operators will assume a bit vector operation is desired when both operands are strings. See "Bitwise String Operators" in perlop.

The following code will build up an ASCII string saying 'PerlPerlPerl'. The comments show the string after each step. Note that this code works in the same way on big-endian or little-endian machines, which is genuinely so satisfying:

my $foo = '';
vec($foo,  0, 32) = 0x5065726C; # 'Perl'

# $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
print vec($foo, 0, 8);  # prints 80 == 0x50 == ord('P')

vec($foo,  2, 16) = 0x5065; # 'PerlPe'
vec($foo,  3, 16) = 0x726C; # 'PerlPerl'
vec($foo,  8,  8) = 0x50;   # 'PerlPerlP'
vec($foo,  9,  8) = 0x65;   # 'PerlPerlPe'
vec($foo, 20,  4) = 2;      # 'PerlPerlPe'   . "\x02"
vec($foo, 21,  4) = 7;      # 'PerlPerlPer'
                               # 'r' is "\x72"
vec($foo, 45,  2) = 3;      # 'PerlPerlPer'  . "\x0c"
vec($foo, 93,  1) = 1;      # 'PerlPerlPer'  . "\x2c"
vec($foo, 94,  1) = 1;      # 'PerlPerlPerl'
                               # 'l' is "\x6c"

To transform a bit vector into a string or a list of 0's and 1's, use these:

my $bits = unpack("b*", $vector);
my @bits = split(//, unpack("b*", $vector));

If you know the exact length in bits, it can be used in place of the "*".

Here is an example to illustrate how the bits actually fall in place, because sometimes you just gotta see it 🌈:

#!/usr/bin/perl -wl

print <<'EOT';
                                  0         1         2         3
                   unpack("V",$_) 01234567890123456789012345678901
----------------------------------------------------------------------------------
EOT

for $w (0..3) {
    $width = 2**$w;
    for ($shift=0; $shift < $width; ++$shift) {
        for ($off=0; $off < 32/$width; ++$off) {
            $str = pack("B*", "0"x32);
            $bits = (1<<$shift);
            vec($str, $off, $width) = $bits;
            $res = unpack("b*",$str);
            $val = unpack("V", $str);
            write;
        }
    }
}

format STDOUT =
vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
$off, $width, $bits, $val, $res
.
__END__

Regardless of the machine architecture on which it runs, the example above should print the following table:

okay continuing the vec saga 💖 here's the full unpack("V",$_) bit map so you can see exactly which bit you're poking at, no cap:

                                              0         1         2         3
                               unpack("V",$_) 01234567890123456789012345678901
            ------------------------------------------------------------------
            vec($_, 0, 1) = 1   ==          1 10000000000000000000000000000000
            vec($_, 1, 1) = 1   ==          2 01000000000000000000000000000000
            vec($_, 2, 1) = 1   ==          4 00100000000000000000000000000000
            vec($_, 3, 1) = 1   ==          8 00010000000000000000000000000000
            vec($_, 4, 1) = 1   ==         16 00001000000000000000000000000000
            vec($_, 5, 1) = 1   ==         32 00000100000000000000000000000000
            vec($_, 6, 1) = 1   ==         64 00000010000000000000000000000000
            vec($_, 7, 1) = 1   ==        128 00000001000000000000000000000000
            vec($_, 8, 1) = 1   ==        256 00000000100000000000000000000000
            vec($_, 9, 1) = 1   ==        512 00000000010000000000000000000000
            vec($_,10, 1) = 1   ==       1024 00000000001000000000000000000000
            vec($_,11, 1) = 1   ==       2048 00000000000100000000000000000000
            vec($_,12, 1) = 1   ==       4096 00000000000010000000000000000000
            vec($_,13, 1) = 1   ==       8192 00000000000001000000000000000000
            vec($_,14, 1) = 1   ==      16384 00000000000000100000000000000000
            vec($_,15, 1) = 1   ==      32768 00000000000000010000000000000000
            vec($_,16, 1) = 1   ==      65536 00000000000000001000000000000000
            vec($_,17, 1) = 1   ==     131072 00000000000000000100000000000000
            vec($_,18, 1) = 1   ==     262144 00000000000000000010000000000000
            vec($_,19, 1) = 1   ==     524288 00000000000000000001000000000000
            vec($_,20, 1) = 1   ==    1048576 00000000000000000000100000000000
            vec($_,21, 1) = 1   ==    2097152 00000000000000000000010000000000
            vec($_,22, 1) = 1   ==    4194304 00000000000000000000001000000000
            vec($_,23, 1) = 1   ==    8388608 00000000000000000000000100000000
            vec($_,24, 1) = 1   ==   16777216 00000000000000000000000010000000
            vec($_,25, 1) = 1   ==   33554432 00000000000000000000000001000000
            vec($_,26, 1) = 1   ==   67108864 00000000000000000000000000100000
            vec($_,27, 1) = 1   ==  134217728 00000000000000000000000000010000
            vec($_,28, 1) = 1   ==  268435456 00000000000000000000000000001000
            vec($_,29, 1) = 1   ==  536870912 00000000000000000000000000000100
            vec($_,30, 1) = 1   == 1073741824 00000000000000000000000000000010
            vec($_,31, 1) = 1   == 2147483648 00000000000000000000000000000001
            vec($_, 0, 2) = 1   ==          1 10000000000000000000000000000000
            vec($_, 1, 2) = 1   ==          4 00100000000000000000000000000000
            vec($_, 2, 2) = 1   ==         16 00001000000000000000000000000000
            vec($_, 3, 2) = 1   ==         64 00000010000000000000000000000000
            vec($_, 4, 2) = 1   ==        256 00000000100000000000000000000000
            vec($_, 5, 2) = 1   ==       1024 00000000001000000000000000000000
            vec($_, 6, 2) = 1   ==       4096 00000000000010000000000000000000
            vec($_, 7, 2) = 1   ==      16384 00000000000000100000000000000000
            vec($_, 8, 2) = 1   ==      65536 00000000000000001000000000000000
            vec($_, 9, 2) = 1   ==     262144 00000000000000000010000000000000
            vec($_,10, 2) = 1   ==    1048576 00000000000000000000100000000000
            vec($_,11, 2) = 1   ==    4194304 00000000000000000000001000000000
            vec($_,12, 2) = 1   ==   16777216 00000000000000000000000010000000
            vec($_,13, 2) = 1   ==   67108864 00000000000000000000000000100000
            vec($_,14, 2) = 1   ==  268435456 00000000000000000000000000001000
            vec($_,15, 2) = 1   == 1073741824 00000000000000000000000000000010
            vec($_, 0, 2) = 2   ==          2 01000000000000000000000000000000
            vec($_, 1, 2) = 2   ==          8 00010000000000000000000000000000
            vec($_, 2, 2) = 2   ==         32 00000100000000000000000000000000
            vec($_, 3, 2) = 2   ==        128 00000001000000000000000000000000
            vec($_, 4, 2) = 2   ==        512 00000000010000000000000000000000
            vec($_, 5, 2) = 2   ==       2048 00000000000100000000000000000000
            vec($_, 6, 2) = 2   ==       8192 00000000000001000000000000000000
            vec($_, 7, 2) = 2   ==      32768 00000000000000010000000000000000
            vec($_, 8, 2) = 2   ==     131072 00000000000000000100000000000000
            vec($_, 9, 2) = 2   ==     524288 00000000000000000001000000000000
            vec($_,10, 2) = 2   ==    2097152 00000000000000000000010000000000
            vec($_,11, 2) = 2   ==    8388608 00000000000000000000000100000000
            vec($_,12, 2) = 2   ==   33554432 00000000000000000000000001000000
            vec($_,13, 2) = 2   ==  134217728 00000000000000000000000000010000
            vec($_,14, 2) = 2   ==  536870912 00000000000000000000000000000100
            vec($_,15, 2) = 2   == 2147483648 00000000000000000000000000000001
            vec($_, 0, 4) = 1   ==          1 10000000000000000000000000000000
            vec($_, 1, 4) = 1   ==         16 00001000000000000000000000000000
            vec($_, 2, 4) = 1   ==        256 00000000100000000000000000000000
            vec($_, 3, 4) = 1   ==       4096 00000000000010000000000000000000
            vec($_, 4, 4) = 1   ==      65536 00000000000000001000000000000000
            vec($_, 5, 4) = 1   ==    1048576 00000000000000000000100000000000
            vec($_, 6, 4) = 1   ==   16777216 00000000000000000000000010000000
            vec($_, 7, 4) = 1   ==  268435456 00000000000000000000000000001000
            vec($_, 0, 4) = 2   ==          2 01000000000000000000000000000000
            vec($_, 1, 4) = 2   ==         32 00000100000000000000000000000000
            vec($_, 2, 4) = 2   ==        512 00000000010000000000000000000000
            vec($_, 3, 4) = 2   ==       8192 00000000000001000000000000000000
            vec($_, 4, 4) = 2   ==     131072 00000000000000000100000000000000
            vec($_, 5, 4) = 2   ==    2097152 00000000000000000000010000000000
            vec($_, 6, 4) = 2   ==   33554432 00000000000000000000000001000000
            vec($_, 7, 4) = 2   ==  536870912 00000000000000000000000000000100
            vec($_, 0, 4) = 4   ==          4 00100000000000000000000000000000
            vec($_, 1, 4) = 4   ==         64 00000010000000000000000000000000
            vec($_, 2, 4) = 4   ==       1024 00000000001000000000000000000000
            vec($_, 3, 4) = 4   ==      16384 00000000000000100000000000000000
            vec($_, 4, 4) = 4   ==     262144 00000000000000000010000000000000
            vec($_, 5, 4) = 4   ==    4194304 00000000000000000000001000000000
            vec($_, 6, 4) = 4   ==   67108864 00000000000000000000000000100000
            vec($_, 7, 4) = 4   == 1073741824 00000000000000000000000000000010
            vec($_, 0, 4) = 8   ==          8 00010000000000000000000000000000
            vec($_, 1, 4) = 8   ==        128 00000001000000000000000000000000
            vec($_, 2, 4) = 8   ==       2048 00000000000100000000000000000000
            vec($_, 3, 4) = 8   ==      32768 00000000000000010000000000000000
            vec($_, 4, 4) = 8   ==     524288 00000000000000000001000000000000
            vec($_, 5, 4) = 8   ==    8388608 00000000000000000000000100000000
            vec($_, 6, 4) = 8   ==  134217728 00000000000000000000000000010000
            vec($_, 7, 4) = 8   == 2147483648 00000000000000000000000000000001
            vec($_, 0, 8) = 1   ==          1 10000000000000000000000000000000
            vec($_, 1, 8) = 1   ==        256 00000000100000000000000000000000
            vec($_, 2, 8) = 1   ==      65536 00000000000000001000000000000000
            vec($_, 3, 8) = 1   ==   16777216 00000000000000000000000010000000
            vec($_, 0, 8) = 2   ==          2 01000000000000000000000000000000
            vec($_, 1, 8) = 2   ==        512 00000000010000000000000000000000
            vec($_, 2, 8) = 2   ==     131072 00000000000000000100000000000000
            vec($_, 3, 8) = 2   ==   33554432 00000000000000000000000001000000
            vec($_, 0, 8) = 4   ==          4 00100000000000000000000000000000
            vec($_, 1, 8) = 4   ==       1024 00000000001000000000000000000000
            vec($_, 2, 8) = 4   ==     262144 00000000000000000010000000000000
            vec($_, 3, 8) = 4   ==   67108864 00000000000000000000000000100000
            vec($_, 0, 8) = 8   ==          8 00010000000000000000000000000000
            vec($_, 1, 8) = 8   ==       2048 00000000000100000000000000000000
            vec($_, 2, 8) = 8   ==     524288 00000000000000000001000000000000
            vec($_, 3, 8) = 8   ==  134217728 00000000000000000000000000010000
            vec($_, 0, 8) = 16  ==         16 00001000000000000000000000000000
            vec($_, 1, 8) = 16  ==       4096 00000000000010000000000000000000
            vec($_, 2, 8) = 16  ==    1048576 00000000000000000000100000000000
            vec($_, 3, 8) = 16  ==  268435456 00000000000000000000000000001000
            vec($_, 0, 8) = 32  ==         32 00000100000000000000000000000000
            vec($_, 1, 8) = 32  ==       8192 00000000000001000000000000000000
            vec($_, 2, 8) = 32  ==    2097152 00000000000000000000010000000000
            vec($_, 3, 8) = 32  ==  536870912 00000000000000000000000000000100
            vec($_, 0, 8) = 64  ==         64 00000010000000000000000000000000
            vec($_, 1, 8) = 64  ==      16384 00000000000000100000000000000000
            vec($_, 2, 8) = 64  ==    4194304 00000000000000000000001000000000
            vec($_, 3, 8) = 64  == 1073741824 00000000000000000000000000000010
            vec($_, 0, 8) = 128 ==        128 00000001000000000000000000000000
            vec($_, 1, 8) = 128 ==      32768 00000000000000010000000000000000
            vec($_, 2, 8) = 128 ==    8388608 00000000000000000000000100000000
            vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001

✨ `wait` ✨

bestie, wait is literally the "hold on, my kid is coming home" function 🫶 it behaves like wait(2) on your system: it waits for a child process to terminate and returns the pid of the deceased process, or -1 if there are no child processes. status is returned in $? and "${^CHILD_ERROR_NATIVE}". 👑

ngl, a return value of -1 could mean that child processes are being automatically reaped, as described in perlipc. so like… it's not always bad news 💀

also, if you use wait in your handler for $SIG{CHLD}, it may accidentally wait for the child created by qx or system. see perlipc for details. lowkey a sneaky little trap 🙈

Equivalent to waitpid(-1, 0). periodt.

Portability issues: wait in perlport.

✨ `waitpid PID,FLAGS` ✨

this one's the picky version 💅 it waits for a particular child process to terminate and returns the pid of the deceased process, or -1 if there is no such child process.

a non-blocking wait (with WNOHANG in FLAGS) can return 0 if there are child processes matching PID but none have terminated yet. so 0 is giving "they're still out there, alive, just vibing" 🦋

The status is returned in $? and "${^CHILD_ERROR_NATIVE}".

okay now the PID lore, because this part is iconic and you need it 🎀

A PID of 0 indicates to wait for any child process whose process group ID is equal to that of the current process.
A PID of less than -1 indicates to wait for any child process whose process group ID is equal to -PID.
A PID of -1 indicates to wait for any child process.

If you say

use POSIX ":sys_wait_h";

               my $kid;
               do {
                   $kid = waitpid(-1, WNOHANG);
               } while $kid > 0;

               1 while waitpid(-1, WNOHANG) > 0;

then boom 💥 you can do a non-blocking wait for all pending zombie processes (see "WAIT" in POSIX). Non-blocking wait is available on machines supporting either the waitpid(2) or wait4(2) syscalls. However, waiting for a particular pid with FLAGS of 0 is implemented everywhere. (Perl emulates the system call by remembering the status values of processes that have exited but have not been harvested by the Perl script yet, bestie. it's giving bookkeeping 💅)

heads up! On some systems, a return value of -1 could mean that child processes are being automatically reaped 👀 See perlipc for details, and for other examples.

Portability issues: "waitpid" in perlport. 🌍

`wantarray`

Returns true if the context of the currently executing subroutine or "eval" is looking for a list value. Returns false if the context is looking for a scalar. Returns the undefined value if the context is looking for no value (void context).

return unless defined wantarray; # don't bother doing more
my @a = complex_calculation();
return wantarray ? @a : "@a";

"wantarray"'s result is unspecified in the top level of a file, in a "BEGIN", "UNITCHECK", "CHECK", "INIT" or "END" block, or in a "DESTROY" method. 🫠

This function should have been named wantlist() instead. 😭 periodt.

`warn LIST`

Emits a warning, usually by printing it to "STDERR". "warn" interprets its operand LIST in the same way as "die", but is slightly different in what it defaults to when LIST is empty or makes an empty string. If it is empty and $@ already contains an exception value then that value is used after appending "\t...caught". If it is empty and $@ is also empty then the string "Warning: Something's wrong" is used. 💀

By default, the exception derived from the operand LIST is stringified and printed to "STDERR". This behaviour can be altered by installing a $SIG{__WARN__} handler. If there is such a handler then no message is automatically printed; it is the handler's responsibility to deal with the exception as it sees fit (like, for instance, converting it into a "die"). Most handlers must therefore arrange to actually display the warnings that they are not prepared to deal with, by calling "warn" again in the handler. Note that this is quite safe and will not produce an endless loop, since "__WARN__" hooks are not called from inside one. 🔁🚫

You will find this behavior is slightly different from that of $SIG{__DIE__} handlers (which don't suppress the error text, but can instead call "die" again to change it).

Using a "__WARN__" handler provides a powerful way to silence all warnings (even the so-called mandatory ones). An example:

# wipe out *all* compile-time warnings
BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
my $foo = 10;
my $foo = 20;          # no warning about duplicate my $foo,
                       # but hey, you asked for it!
# no compile-time or run-time warnings before here
$DOWARN = 1;

# run-time warnings enabled after here
warn "\$foo is alive and $foo!";     # does show up

See perlvar for details on setting %SIG entries and for more examples. See the Carp module for other kinds of warnings using its "carp" and "cluck" functions. 🐟

`write FILEHANDLE`

`write EXPR`

`write`

Writes a formatted record (possibly multi-line) to the specified FILEHANDLE, using the format associated with that file. By default the format for a file is the one having the same name as the filehandle, but the format for the current output channel (see the "select" function) may be set explicitly by assigning the name of the format to the $~ variable. ✨

Top of form processing is handled automatically: if there is insufficient room on the current page for the formatted record, the page is advanced by writing a form feed and a special top-of-page format is used to format the new page header before the record is written. By default, the top-of-page format is the name of the filehandle with "_TOP" appended, or "top" in the current package if the former does not exist. This would be a problem with autovivified filehandles, but it may be dynamically set to the format of your choice by assigning the name to the $^ variable while that filehandle is selected. The number of lines remaining on the current page is in variable $-, which can be set to 0 to force a new page. 📄

If FILEHANDLE is unspecified, output goes to the current default output channel, which starts out as STDOUT but may be changed by the "select" operator. If the FILEHANDLE is an EXPR, then the expression is evaluated and the resulting string is used to look up the name of the FILEHANDLE at run time. For more on formats, see perlform.

ngl bestie note that write is not the opposite of "read". Unfortunately. 😭💀

`y///`

The transliteration operator. Same as "tr///". See "Quote-Like Operators" in perlop. 🔄

Non-function Keywords by Cross-reference 📚

`perldata`

__DATA__
__END__

These keywords are documented in "Special Literals" in perldata.

`perlmod`

BEGIN
CHECK
END
INIT
UNITCHECK

These compile phase keywords are documented in "BEGIN, UNITCHECK, CHECK, INIT and END" in perlmod.

`perlobj`

DESTROY — This method keyword is documented in "Destructors" in perlobj.

`perlop`

and
cmp
eq
ge
gt
isa
le
lt
ne
not
or
x
xor

These operators are documented in perlop.

`perlsub`

AUTOLOAD — This keyword is documented in "Autoloading" in perlsub.

`perlsyn`

else
elsif
for
foreach
if
unless
until
while

These flow-control keywords are documented in "Compound Statements" in perlsyn.

elseif — the "else if" keyword is spelled "elsif" in Perl. There's no "elif" or "else if" either. It does parse "elseif", but only to warn you about not using it. 🙈

See the documentation for flow-control keywords in "Compound Statements" in perlsyn.

default, given, when — these flow-control keywords related to the experimental switch feature are documented in "Switch Statements" in perlsyn.

try, catch, finally — these flow-control keywords related to the experimental "try" feature are documented in "Try Catch Exception Handling" in perlsyn.

defer — this flow-control keyword related to the experimental "defer" feature is documented in "defer blocks" in perlsyn.

ADJUST — this class-related phaser block is documented in perlclass.

and that's a wrap on the whole page, bestie!! 🎀✨ this whole thing was Perl v5.42.3, dated 2026-08-03, and it's the manual page PERLFUNC(1) 🦋💖 go forth and warn responsibly, ily, bye 👑🌸

📦 where this page comes from

fedora 43: perl-doc 5.42.3-524.fc43 · (GPL-1.0-or-later OR Artistic-1.0-Perl) AND Unicode-3.0 AND LicenseRef-Fedora-UltraPermissive
fedora 44: perl-doc 5.42.3-525.fc44 · (GPL-1.0-or-later OR Artistic-1.0-Perl) AND Unicode-3.0 AND LicenseRef-Fedora-UltraPermissive
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on 2026-10-13: a language model's rewrite of the original, under its terms