PERLVAR(1)
🌸 PERLVAR 🌸
okay bestie, gather round, because this is the page: perlvar — Perl's predefined variables. every magic lil $ and @ and % that Perl just hands you, no declaration, no nothing. it's giving "the universe already set these up for you." periodt. 💅
✨ NAME ✨
perlvar — Perl predefined variables 🫶
💖 DESCRIPTION 💖
🎀 The Syntax of Variable Names 🎀
so like, variable names in Perl can come in several formats and you need to know the rules, no cap.
Format one: the normal girlie. Usually they start with a letter or an underscore, and then they can be arbitrarily long (up to an internal limit of 251 characters) and can contain letters, digits, underscores, or the special sequence "::" or "'". In that case, the part before the last "::" or "'" is taken to be a package qualifier; see perlmod. ✨
⚠️ but here's the tea: a Unicode letter that is not ASCII is not considered to be a letter unlessuse utf8is in effect, and somewhat more complicated rules apply; see"Identifier parsing" in perldatafor details.
Format two: the chaotic ones. Perl variable names may also be:
- a sequence of digits,
- a single punctuation character, or
- the two-character sequence:
"^"(caret, aka CIRCUMFLEX ACCENT) followed by any one of the characters"[][A-Z^_?\]".
These names are all reserved for special uses by Perl; for example, the all-digits names are used to hold data captured by backreferences after a regular expression match. iykyk. 🧚
Format three: the caret-alphanumeric girlies. Since Perl v5.6.0, Perl variable names may also be alphanumeric strings preceded by a caret. These must all be written using the demarcated variable form using curly braces such as ${^Foo}; the braces are not optional. ${^Foo} denotes the scalar variable whose name is considered to be a control-"F" followed by two "o"'s. (See "Demarcated variable names using braces" in perldata for more information on this form of spelling a variable name or specifying access to an element of an array or a hash.) These variables are reserved for future special uses by Perl, except for the ones that begin with "^_" (caret-underscore). 💀
No name that begins with "^_" will acquire a special meaning in any future version of Perl; such names may therefore be used safely in programs. $^_ itself, however, is reserved. 🙈
⚠️ also bestie, note that you also must use the demarcated form to access subscripts of variables of this type when interpolating. for instance, to access the first element of the"@{^CAPTURE}"variable inside of a double quoted string you would write${^CAPTURE[0]}and NOT${^CAPTURE}[0]— which would mean to reference a scalar variable named"${^CAPTURE}"and not index 0 of the magic"@{^CAPTURE}"array which is populated by the regex engine. this trips people up SO bad, don't let it be you 💅
Perl identifiers that begin with digits or punctuation characters are exempt from the effects of the "package" declaration and are always forced to be in package "main"; they are also exempt from "strict 'vars'" errors. A few other names are also exempt in these ways:
ENV STDIN
INC STDOUT
ARGV STDERR
ARGVOUT
SIGIn particular, the special "${^_XYZ}" variables are always taken to be in package "main", regardless of any "package" declarations presently in scope. iconic behaviour honestly. 👑
🔥 SPECIAL VARIABLES 🔥
The following names have special meaning to Perl. Most punctuation names have reasonable mnemonics, or analogs in the shells. Nevertheless, if you wish to use long variable names, you need only say:
use English;at the top of your program. This aliases all the short names to the long names in the current package. Some even have medium names, generally borrowed from awk. For more info, please see English. 🌈
Before you continue, note the sort order for variables. In general, we first list the variables in case-insensitive, almost-lexicographical order (ignoring the "{" or "^" preceding words, as in "${^UNICODE}" or $^T), although $_ and @_ move up to the top of the pile. For variables with the same identifier, we list it in order of scalar, array, hash, and bareword. yes it's a whole system, we're organised here 💖
⭐ General Variables ⭐
$ARG,$_— The default input and pattern-searching space. The following pairs are equivalent:
while (<>) {...} # equivalent only in while!
while (defined($_ = <>)) {...}
/^Subject:/
$_ =~ /^Subject:/
tr/a-z/A-Z/
$_ =~ tr/a-z/A-Z/
chomp
chomp($_)Here are the places where Perl will assume $_ even if you don't use it:
- The following functions use
$_as a default argument:
abs, alarm, chomp, chop, chr, chroot, cos, defined, eval, evalbytes, exp, fc, glob, hex, int, lc, lcfirst, length, log, lstat, mkdir, oct, ord, pos, print, printf, quotemeta, readlink, readpipe, ref, require, reverse (in scalar context only), rmdir, say, sin, split (for its second argument), sqrt, stat, study, uc, ucfirst, unlink, unpack.
- All file tests (
"-f","-d") except for"-t", which defaults toSTDIN. See"-X" in perlfunc. - The pattern matching operations
"m//","s///"and"tr///"(aka"y///") when used without an"=~"operator. - The default iterator variable in a
"foreach"loop if no other variable is supplied. - The implicit iterator variable in the
grep()andmap()functions. - The implicit variable of
given(). - The default place to put the next value or input record when a
"<FH>","readline","readdir"or"each"operation's result is tested by itself as the sole criterion of a"while"test. Outside a"while"test, this will not happen.
$_ is a global variable.
However, between perl v5.10.0 and v5.24.0, it could be used lexically by writing "my $_". Making $_ refer to the global $_ in the same scope was then possible with "our $_". This experimental feature was removed and is now a fatal error, but you may encounter it in older code. 💀
Mnemonic: underline is understood in certain operations.
@ARG,@_— Within a subroutine the array@_contains the parameters passed to that subroutine. Inside a subroutine,@_is the default array for the array operators"pop"and"shift".
See perlsub. 🫶
$LIST_SEPARATOR,$"— When an array or an array slice is interpolated into a double-quoted string or a similar context such as"/.../", its elements are separated by this value. Default is a space. For example, this:
print "The array is: @array\n";is equivalent to this:
print "The array is: " . join($", @array) . "\n";Mnemonic: works in double-quoted context.
$PROCESS_ID,$PID,$$— The process number of the Perl running this script. Though you can set this variable, doing so is generally discouraged, although it can be invaluable for some testing purposes. It will be reset automatically acrossfork()calls.
💀 Note for Linux and Debian GNU/kFreeBSD users: Before Perl v5.16.0 perl would emulate POSIX semantics on Linux systems using LinuxThreads, a partial implementation of POSIX Threads that has since been superseded by the Native POSIX Thread Library (NPTL).
LinuxThreads is now obsolete on Linux, and caching getpid() like this made embedding perl unnecessarily complex (since you'd have to manually update the value of $$), so now $$ and getppid() will always return the same values as the underlying C library.
Debian GNU/kFreeBSD systems also used LinuxThreads up until and including the 6.0 release, but after that moved to FreeBSD thread semantics, which are POSIX-like.
To see if your system is affected by this discrepancy check if getconf GNU_LIBPTHREAD_VERSION | grep -q NPTL returns a false value. NTPL threads preserve the POSIX semantics.
Mnemonic: same as shells. 🐚
$PROGRAM_NAME,$0— Contains the name of the program being executed.
On some (but not all) operating systems assigning to $0 modifies the argument area that the "ps" program sees. On some platforms you may have to use special "ps" options or a different "ps" to see the changes. Modifying the $0 is more useful as a way of indicating the current program state than it is for hiding the program you're running. 👑
Note that there are platform-specific limitations on the maximum length of $0. In the most extreme case it may be limited to the space occupied by the original $0.
In some platforms there may be arbitrary amount of padding, for example space characters, after the modified name as shown by "ps". In some platforms this padding may extend all the way to the original length of the argument area, no matter what you do (this is the case for example with Linux 2.2). 😭
Note for BSD users: setting $0 does not completely remove "perl" from the ps(1) output. For example, setting $0 to "foobar" may result in "perl: foobar (perl)" (whether both the "perl: " prefix and the " (perl)" suffix are shown depends on your exact BSD variant and version). This is an operating system feature, Perl cannot help it. 💀
In multithreaded scripts Perl coordinates the threads so that any thread may modify its copy of the $0 and the change becomes visible to ps(1) (assuming the operating system plays along). Note that the view of $0 the other threads have will not change since they have their own copies of it.
If the program has been given to perl via the switches "-e" or "-E", $0 will contain the string "-e".
On Linux as of perl v5.14.0 the legacy process name will be set with prctl(2), in addition to altering the POSIX name via "argv[0]" as perl has done since version 4.000. Now system utilities that read the legacy process name such as ps, top and killall will recognize the name you set when assigning to $0. The string you supply will be cut off at 16 bytes — this is a limitation imposed by Linux. ✂️
⚠️ Wide characters are invalid in $0 values. For historical reasons, though, Perl accepts them and encodes them to UTF-8. When this happens a wide-character warning is triggered.
Mnemonic: same as sh and ksh.
$REAL_GROUP_ID,$GID,$(— The real gid of this process. If you are on a machine that supports membership in multiple groups simultaneously, gives a space separated list of groups you are in. The first number is the one returned bygetgid(), and the subsequent ones bygetgroups(), one of which may be the same as the first number. 🌸
Okay so real talk about $( — whatever you assign to it has to be one single number, and that number sets the real gid. Which means you should not take the value you got out of $( and just yeet it back in there. Not unless you force it numeric first, like by adding zero. Because $( is built different from the effective gid ($)), and $) does take a list. Not the same girl, bestie. 💅
You can change both the real gid and the effective gid at the same time by using POSIX::setgid(). And heads up: changes to $( require you to check $! after the attempt, so you can catch any possible errors.
Mnemonic: parentheses are used to group things. The real gid is the group you left, if you're running setgid.
🌸 $EFFECTIVE_GROUP_ID, $EGID, $) 🌸
The effective gid of this process. If you're on a machine that supports membership in multiple groups simultaneously, this gives you a space separated list of groups you're in. The first number is the one returned by getegid(), and the subsequent ones by getgroups(), one of which may be the same as the first number. (Yes, they can overlap. It's giving redundancy but we move.)
Similarly, a value assigned to $) must also be a space-separated list of numbers. The first number sets the effective gid, and the rest (if any) are passed to setgroups(). To get the effect of an empty list for setgroups(), just repeat the new effective gid — that is, to force an effective gid of 5 and an effectively empty setgroups() list, say $) = "5 5". Iconic workaround, honestly.
You can change both the effective gid and the real gid at the same time by using POSIX::setgid() (use only a single numeric argument). Changes to $) require a check to $! to detect any possible errors after an attempted change. 🫶
$<, $>, $( and $) can be set only on machines that support the corresponding set[re][ug]id() routine. And $( and $) can be swapped only on machines supporting setregid().
Mnemonic: parentheses are used to group things. The effective gid is the group that's right for you, if you're running setgid.
💜 $REAL_USER_ID, $UID, $< 💜
The real uid of this process. You can change both the real uid and the effective uid at the same time by using POSIX::setuid(). Since changes to $< require a system call, check $! after a change attempt to detect any possible errors. Fr fr, don't skip the check. 😭
Mnemonic: it's the uid you came from, if you're running setuid.
👑 $EFFECTIVE_USER_ID, $EUID, $> 👑
The effective uid of this process. For example:
$< = $>; # set real to effective uid
($<,$>) = ($>,$<); # swap real and effective uidsYou can change both the effective uid and the real uid at the same time by using POSIX::setuid(). Changes to $> require a check to $! to detect any possible errors after an attempted change.
$< and $> can be swapped only on machines supporting setreuid().
Mnemonic: it's the uid you went to, if you're running setuid.
🎀 $SUBSCRIPT_SEPARATOR, $SUBSEP, $; 🎀
The subscript separator for multidimensional array emulation. So if you refer to a hash element as
$foo{$x,$y,$z}it really means
$foo{join($;, $x, $y, $z)}But don't put
@foo{$x,$y,$z} # a slice--note the @which means
($foo{$x},$foo{$y},$foo{$z})Different vibe entirely. Note the @. 👀
Default is "\034", the same as SUBSEP in awk. If your keys contain binary data there might not be any safe value for $;.
Consider using "real" multidimensional arrays as described in perllol.
Mnemonic: comma (the syntactic subscript separator) is a semi-semicolon.
🦋 $a, $b 🦋
Special package variables when using sort(), see "sort" in perlfunc. Because of this specialness $a and $b don't need to be declared (using "use vars", or our()) even when using the "strict 'vars'" pragma. And don't lexicalize them with "my $a" or "my $b" if you want to be able to use them in the sort() comparison block or function. That's the whole point of them, bestie. 🙈
🌈 %ENV 🌈
The hash %ENV contains your current environment. Setting a value in "ENV" changes the environment for any child processes you subsequently fork() off.
As of v5.18.0, both keys and values stored in %ENV are stringified.
my $foo = 1;
$ENV{'bar'} = \$foo;
if( ref $ENV{'bar'} ) {
say "Pre 5.18.0 Behaviour";
} else {
say "Post 5.18.0 Behaviour";
}Previously, only child processes received stringified values:
my $foo = 1;
$ENV{'bar'} = \$foo;
# Always printed 'non ref'
system($^X, '-e',
q/print ( ref $ENV{'bar'} ? 'ref' : 'non ref' ) /);This happens because you can't really share arbitrary data structures with foreign processes. Like, you just can't. 💀
⭐ $OLD_PERL_VERSION, $] ⭐
The revision, version, and subversion of the Perl interpreter, represented as a decimal of the form 5.XXXYYY, where XXX is the version / 1e3 and YYY is the subversion / 1e6. For example, Perl v5.10.1 would be "5.010001".
This variable can be used to determine whether the Perl interpreter executing a script is in the right range of versions:
warn "No PerlIO!\n" if "$]" < 5.008;When comparing $], numeric comparison operators should be used, but the variable should be stringified first to avoid issues where its original numeric value is inaccurate. Sneaky little detail, don't sleep on it.
See also the documentation of "use VERSION" and "require VERSION" for a convenient way to fail if the running Perl interpreter is too old.
See "$^V" for a representation of the Perl version as a version object, which allows more flexible string comparisons.
The main advantage of $] over $^V is that it works the same on any version of Perl. The disadvantages are that it can't easily be compared to versions in other formats (e.g. literal v-strings, "v1.2.3" or version objects) and numeric comparisons are subject to the binary floating point representation; it's good for numeric literal version checks and bad for comparing to a variable that hasn't been sanity-checked.
The $OLD_PERL_VERSION form was added in Perl v5.20.0 for historical reasons but its use is discouraged. (If your reason to use $] is to run code on old perls then referring to it as $OLD_PERL_VERSION would be self-defeating.) 💀
Mnemonic: Is this version of perl in the right bracket?
✨ $SYSTEM_FD_MAX, $^F ✨
The maximum system file descriptor, ordinarily 2. System file descriptors are passed to exec()ed processes, while higher file descriptors are not. Also, during an open(), system file descriptors are preserved even if the open() fails (ordinary file descriptors are closed before the open() is attempted). The close-on-exec status of a file descriptor will be decided according to the value of $^F when the corresponding file, pipe, or socket was opened, not the time of the exec(). Timing matters, bestie. ⏰
🍓 @F 🍓
The array @F contains the fields of each line read in when autosplit mode is turned on. See perlrun for the -a switch. This array is package-specific, and must be declared or given a full package name if not in package main when running under "strict 'vars'".
💖 @INC 💖
The array @INC contains the list of places that the "do EXPR", "require", or "use" constructs look for their library files. It initially consists of the arguments to any -I command-line switches, followed by the default Perl library, probably /usr/local/lib/perl. Prior to Perl 5.26, "." -which represents the current directory, was included in @INC; it has been removed. This change in behavior is documented in "PERL_USE_UNSAFE_INC" and it is not recommended that "." be re-added to @INC. If you need to modify @INC at runtime, you should use the "use lib" pragma to get the machine-dependent library properly loaded as well:
use lib '/mypath/libdir/';
use SomeMod;You can also insert hooks into the file inclusion system by putting Perl code directly into @INC. Those hooks may be subroutine references, array references or blessed objects. See "require" in perlfunc for details.
🧚 %INC 🧚
The hash %INC contains entries for each filename included via the "do", "require", or "use" operators. The key is the filename you specified (with module names converted to pathnames), and the value is the location of the file found. The "require" operator uses this hash to determine whether a particular file has already been included.
If the file was loaded via a hook (e.g. a subroutine reference, see "require" in perlfunc for a description of these hooks), this hook is by default inserted into %INC in place of a filename. Note, however, that the hook may have set the %INC entry by itself to provide some more specific info. Self-aware hook, we love to see it. 🫶
🌸 $INC 🌸
As of 5.37.7 when an @INC hook is executed the index of the @INC array that holds the hook will be localized into the $INC variable. When the hook returns the integer successor of its value will be used to determine the next index in @INC that will be checked, thus if it is set to -1 (or "undef") the traversal over the @INC array will be restarted from its beginning.
Normally traversal through the @INC array is from beginning to end ("0 .. $#INC"), and if the @INC array is modified by the hook the iterator may be left in a state where newly added entries are skipped. Changing this value allows an @INC hook to rewrite the @INC array and tell Perl where to continue afterwards. See "require" in perlfunc for details on @INC hooks.
💅 $INPLACE_EDIT, $^I 💅
The current value of the inplace-edit extension. Use "undef" to disable inplace editing.
Mnemonic: value of -i switch.
🔥 @ISA 🔥
Each package contains a special array called @ISA which contains a list of that class's parent classes, if any. This array is simply a list of scalars, each of which is a string that corresponds to a package name. The array is examined when Perl does method resolution, which is covered in perlobj.
Okay so if you wanna load packages and drop them into @ISA at the same time, that's the parent pragma's whole thing. The discouraged base pragma also does this, but bestie, don't touch it unless you literally need compatibility with the equally discouraged fields pragma. That's the only excuse. 💀
💖 $^M — the emergency snack stash
By default, running out of memory is an untrappable, fatal error. Like, you can't catch it, you can't sweet-talk it, it's just over. BUT — if your Perl was suitably built, it can reach into $^M and use whatever's in there as an emergency memory pool after it die()s. A lil backup battery, basically. 🔋
Say your Perl was compiled with -DPERL_EMERGENCY_SBRK and it's using Perl's own malloc. Then this:
$^M = 'a' x (1 << 16);would allocate a 64K buffer for use in an emergency. Slay. 💅
Wanna know how to add custom C compilation flags when you compile perl? Go read the INSTALL file in the Perl distribution. And because they really don't want you casually playing with this advanced feature, there is no English long name for this variable. You gotta earn it. 😤
This variable was added in Perl 5.004. ✨
🌸 ${^MAX_NESTED_EVAL_BEGIN_BLOCKS} — the nesting bouncer
This one decides the maximum number of eval EXPR/BEGIN or require/BEGIN block nesting that's allowed. Which means — plot twist — it also controls the maximum nesting of use statements as well, since use is basically require in a trench coat. 👑
The default is 1000, which should be sufficiently large for normal working purposes. But if you absolutely must raise it, be conservative with your choice, because otherwise you may hit segfaults from exhaustion of the C stack. It seems unlikely that real code has a use depth above 1000, but they left it configurable just in case. Cute of them. 🫶
Set it to 0 and BEGIN blocks inside of eval EXPR or require EXPR are forbidden entirely — they'll trigger an exception that terminates the compilation, and then:
- in the case of
require, it throws an exception - in the case of
eval, it returns the error in$@as usual
Now consider this lil piece of chaos:
perl -le'sub f { eval "BEGIN { f() }"; } f()'Every single invocation of f() eats up considerable C stack, and this variable exists to make code like this die instead of exhausting the C stack and triggering a segfault. Needless to say, code like this is unusual, and it's unlikely you'll actually need to raise the setting. But it may be useful to set it to 0 for a limited time period to prevent BEGIN{} blocks from being executed during an eval EXPR. Situational queen. 💅
Heads up bestie: setting this to 1 would NOT affect code like this:
BEGIN { $n += 1; BEGIN { $n += 2; BEGIN { $n += 4 } } }Why not? Because BEGIN blocks are executed immediately after they're completed, so the innermost one runs before the ones containing it have even finished compiling — and the depth never goes above 1. In fact, that code up there is equivalent to:
BEGIN { $n+=4 }
BEGIN { $n+=2 }
BEGIN { $n+=1 }which makes it obvious why a ${^MAX_EVAL_BEGIN_DEPTH} of 1 would not block this code.
Only BEGIN's executed inside of an eval or require (possibly via use) are affected. Periodt.
🦋 $OSNAME, $^O — the "what am I even running on" twins
The name of the operating system under which this copy of Perl was built, as determined during the configuration process. For examples, see "PLATFORMS" in perlport. 📖
The value is identical to $Config{'osname'}. See also Config and the -V command-line switch documented in perlrun.
Now on Windows platforms, $^O is not very helpful: since it's always MSWin32, it doesn't tell the difference between 95/98/ME/NT/2000/XP/CE/.NET. So if you need to tell those variants apart, use Win32::GetOSName() or Win32::GetOSVersion() (see Win32 and perlport). Windows stays keeping secrets, I guess. 🙈
This variable was added in Perl 5.003. ⭐
💜 %SIG — the signal handler group chat
The hash %SIG contains signal handlers for signals. Like so:
sub handler { # 1st argument is signal name
my($sig) = @_;
print "Caught a SIG$sig--shutting down\n";
close(LOG);
exit(0);
}
$SIG{'INT'} = \&handler;
$SIG{'QUIT'} = \&handler;
...
$SIG{'INT'} = 'DEFAULT'; # restore default action
$SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUITUsing a value of 'IGNORE' usually has the effect of ignoring the signal — except for the CHLD signal. See perlipc for more about this special case. And using an empty string or undef as the value has the same effect as 'DEFAULT'.
Here are some other examples, for the girlies:
$SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not
# recommended)
$SIG{"PIPE"} = \&Plumber; # just fine; assume current
# Plumber
$SIG{"PIPE"} = *Plumber; # somewhat esoteric
$SIG{"PIPE"} = Plumber(); # oops, what did Plumber()
# return??Be sure not to use a bareword as the name of a signal handler, lest you inadvertently call it. That last example up there? That's the oops. 😭
Using a string that doesn't correspond to any existing function, or a glob that doesn't contain a code slot, is equivalent to 'IGNORE' — but a warning is emitted when the handler is being called. (That warning is not emitted for the internal hooks described below, fyi.)
If your system has the sigaction() function, then signal handlers are installed using it. Which means you get reliable signal handling. Slay. ✨
The default delivery policy of signals changed in Perl v5.8.0 from immediate (also known as "unsafe") to deferred, also known as "safe signals". See perlipc for more information.
the internal hooks 🪝
Certain internal hooks can also be set using the %SIG hash.
The routine indicated by $SIG{__WARN__} is called when a warning message is about to be printed. The warning message is passed as the first argument. The presence of a __WARN__ hook causes the ordinary printing of warnings to STDERR to be suppressed. You can use this to save warnings in a variable, or turn warnings into fatal errors, like this:
local $SIG{__WARN__} = sub { die $_[0] };
eval $proggie;As the 'IGNORE' hook is not supported by __WARN__, its effect is the same as using 'DEFAULT'. You can disable warnings using the empty subroutine:
local $SIG{__WARN__} = sub {};The routine indicated by $SIG{__DIE__} is called when a fatal exception is about to be thrown. The error message is passed as the first argument. When a __DIE__ hook routine returns, the exception processing continues as it would have in the absence of the hook — unless the hook routine itself throws an exception. The __DIE__ handler is explicitly disabled during the call, so that you can die from a __DIE__ handler. Similarly for __WARN__. 🫶
The $SIG{__DIE__} hook is called even inside an eval(). It was never intended to happen this way, but an implementation glitch made this possible. This used to be deprecated, since it allowed strange action at a distance like rewriting a pending exception in $@. Plans to rectify this have been scrapped, because users found that rewriting a pending exception is actually a useful feature, and not a bug. The people have spoken. 💅
The $SIG{__DIE__} hook doesn't support 'IGNORE'; it has the same effect as 'DEFAULT'.
__DIE__/__WARN__ handlers are very special in one respect: they may be called to report (probable) errors found by the parser. In such a case the parser may be in an inconsistent state, so any attempt to evaluate Perl code from such a handler will probably result in a segfault. This means that warnings or errors that result from parsing Perl should be used with extreme caution, like this:
require Carp if defined $^S;
Carp::confess("Something wrong") if defined &Carp::confess;
die "Something wrong, but could not load Carp to give "
. "backtrace...\n\t"
. "To see backtrace try starting Perl with -MCarp switch";Here the first line will load Carp unless it is the parser who called the handler. The second line will print backtrace and die if Carp was available. The third line will be executed only if Carp was not available. A lil if/elsif/else chain but make it chaos. 🎀
Having to even think about the $^S variable in your exception handlers is simply wrong. $SIG{__DIE__} as currently implemented invites grievous and difficult to track down errors. Avoid it and use an END{} or CORE::GLOBAL::die override instead. That's the vibe check, and $SIG{__DIE__} did not pass. 💀
See "die" in perlfunc, "warn" in perlfunc, "eval" in perlfunc, and warnings for additional information.
🧚 %{^HOOK} — the keyword hook era
This hash contains coderefs which are called when various perl keywords which are hard or impossible to wrap are called. The keys of this hash are named after the keyword that is being hooked, followed by two underbars and then a phase term; either before or after. 🪝
Perl will throw an error if you attempt to modify a key which is not documented to exist, or if you attempt to store anything other than a code reference or undef in the hash. If you wish to use an object to implement a hook, you can use currying to embed the object into an anonymous code reference. Fancy. 💅
Currently there is only one keyword which can be hooked, require, but it is expected that in future releases there will be additional keywords with hook support. Stay tuned, bestie. 👑
require__before
The routine indicated by ${^HOOK}{require__before} is called by require before it checks %INC, looks up @INC, calls INC hooks, or compiles any code. It is called with a single argument, the filename for the item being required (package names are converted to paths). It may alter this filename to change what file is loaded. If the hook dies during execution, then it will block the require from executing. Gatekeeping, but make it code. 🚪
Good code returns a "post-action" coderef from the require__before hook when it wants to do something with shared state both before and after the require keyword actually ran. That coderef gets executed once the require finishes up — and bestie, it runs no matter what, whether the require succeeded or threw an exception 💀. It gets called with the filename that was required, and you can peek at %INC to figure out if the require actually worked. Anything else the require__before hook returns? Silently ignored. Just like that. 🙈
require__before hooks fire in FIFO order, and if a hook hands back a code reference, those code references fire in FILO order. So like — if A requires B requires C, then require__before runs first for A, then B, then C, and the post-action code reference runs first for C, then B, and finally A. Stack behaviour, we love to see it ✨
Well behaved code should make sure that when it installs a require__before hook, any hook that was there before still gets called, and that their return value — if it's a code reference — gets called too. Peep require in perlfunc for an example implementation.
🌸 require__after 🌸
The routine hanging out at ${^HOOK}{require__after} gets called by require after the require completes. One single argument: the filename of the item being required (package names get converted to paths, cute). It runs when the require completes — either by exception or by just finishing the require statement — and yeah, you can check %INC to see if it was a success or a whole mess 😭.
The require__after hook is called for each required file in FILO order. So if A requires B requires C, require__after runs first for C, then B, then A. Last in, first out, main character energy 💅
💜 $BASETIME / $^T 💜
The time the program started running, in seconds since the epoch (aka the beginning of 1970). The values that -M, -A, and -C filetests hand back are all based on this one. 🕰️
⭐ $PERL_VERSION / $^V ⭐
The revision, version, and subversion of the Perl interpreter, chillin' as a version object.
This variable first showed up in perl v5.6.0 — earlier perls will just see an undefined value, rip. And before perl v5.10.0, $^V was a v-string instead of a version object.
You can use $^V to figure out whether the Perl interpreter running your script is in the right version range. Like:
warn "Hashes not randomized!\n" if !$^V or $^V lt v5.8.1Version objects overload stringification, but if you want to portably turn $^V into its string representation, use sprintf()'s "%vd" conversion — works for both v-strings and version objects, iconic:
printf "version is v%vd\n", $^V; # Perl's versionCheck the docs for use VERSION and require VERSION if you want a convenient way to fail when the running Perl interpreter is too old.
Also see $] for a decimal representation of the Perl version.
The main glow-up of $^V over $] is that, for Perl v5.10.0 or later, it overloads operators, so comparing it against other version representations is easy-peasy (decimal, literal v-string, "v1.2.3", or objects). The downside? Before v5.10.0 it was only a literal v-string, which can't be easily printed or compared — whereas $] behaves the same on every version of Perl. Trade-offs, bestie.
Mnemonic: use ^V for a version object. 🧠
🦋 $EXECUTABLE_NAME / $^X 🦋
The name used to execute the current copy of Perl, pulled from C's argv[0] or (where it's supported) /proc/self/exe.
Depending on the host operating system, $^X might be a relative or absolute pathname of the perl program file, or it might just be the string used to invoke perl without being the pathname of the perl program file at all. Also, most operating systems let you invoke programs that aren't even in the PATH environment variable, so there's no guarantee that $^X lives in PATH. And for VMS, the value may or may not include a version number. Chaos, honestly 🙃
Usually you can use $^X to re-invoke an independent copy of the same perl that's currently running, like:
@first_run = `$^X -le "print int rand 100 for 1..100"`;But remember — not every operating system supports forking or capturing command output, so this complex little statement may not be portable.
It is not safe to use the value of$^Xas a path name of a file! Some operating systems have a mandatory suffix on executable files but don't require that suffix when you invoke a command. To convert$^Xinto a path name, use these statements:
# Build up a set of file names (not command names).
use Config;
my $this_perl = $^X;
if ($^O ne 'VMS') {
$this_perl .= $Config{_exe}
unless $this_perl =~ m/$Config{_exe}$/i;
}And because a whole lot of operating systems let anyone with read access to the Perl program file copy it, patch it, and run the copy, a security-conscious Perl programmer should be careful to invoke the installed copy of perl — not the copy $^X is pointing at. These statements get that done, and produce a pathname you can invoke as a command or reference as a file:
use Config;
my $secure_perl_path = $Config{perlpath};
if ($^O ne 'VMS') {
$secure_perl_path .= $Config{_exe}
unless $secure_perl_path =~ m/$Config{_exe}$/i;
}🌈 Variables related to regular expressions 🌈
Most of the special variables tied to regular expressions are side effects. Perl sets them once it's completed a match successfully, so you should always check the match result before you go using them. For instance:
if( /P(A)TT(ER)N/ ) {
print "I found $1 and $2\n";
}These variables are read-only, and they behave like a dynamically scoped variable — with only a few exceptions, which are explicitly documented as behaving otherwise. Scroll on for the deets. 👀
✨ Scoping Rules of Regex Variables ✨
Regex variables let you access the state of the most recent successful regex match in the current dynamic scope. The variables themselves are global and unscoped, but the data they access is scoped kind of like dynamically scoped variables — every successful match acts as if it localizes a global state object to the current block or file scope. (See "Compound Statements" in perlsyn for more on dynamic scoping and the local keyword.)
A successful match includes any successful match done by the search and replace operator s///, plus the ones done by the m// operator.
Consider this code:
my @state;
sub matchit {
push @state, $1; # pushes "baz"
my $str = shift;
$str =~ /(zat)/; # matches "zat"
push @state, $1; # pushes "zat"
}
{
$str = "foo bar baz blorp zat";
$str =~ /(foo)/; # matches "foo"
push @state, $1; # pushes "foo"
{
$str =~ /(pizza)/; # does NOT match
push @state, $1; # pushes "foo"
$str =~ /(bar)/; # matches "bar"
push @state, $1; # pushes "bar"
$str =~ /(baz)/; # matches "baz"
matchit($str); # see above
push @state, $1; # pushes "baz"
}
$str =~ s/noodles/rice/; # does NOT match
push @state, $1; # pushes "foo"
$str =~ s/(blorp)/zwoop/; # matches "blorp"
push @state, $1; # pushes "blorp"
}
# the following prints "foo, foo, bar, baz, zat, baz, foo, blorp"
print join ",", @state;Notice how each successful match in the exact same scope overrides the match context of the previous successful match — but unsuccessful matches don't. Also note that in an inner nested scope, the previous state from an outer dynamic scope sticks around until it gets overridden by another successful match, and when the inner nested scope exits, whatever match context was in effect before the inner successful match gets restored when the scope wraps up. Delulu? No, that's just how it works. 💫
Known issue:goto LABELmay interact poorly with the dynamically scoped match context. This might not be fixable, and it's considered one of many very good reasons to avoidgoto LABEL.
🔥 Performance issues 🔥
Traditionally in Perl, any use of any of the three variables $``, $&or$'(or theiruse English` equivalents) anywhere in the code caused all subsequent successful pattern matches to make a copy of the matched string — just in case the code might access one of those variables later. That put a considerable performance penalty across the entire program, which is why using these variables has generally been discouraged. Not the vibe. 😭
In Perl 5.6.0 the @- and @+ dynamic arrays showed up, supplying the indices of successful matches. So you could do this instead:
$str =~ /pattern/;
print $`, $&, $'; # bad: performance hit
print # good: no performance hit
substr($str, 0, $-[0]),
substr($str, $-[0], $+[0]-$-[0]),
substr($str, $+[0]);In Perl 5.10.0 the /p match operator flag and the ${^PREMATCH}, ${^MATCH}, and ${^POSTMATCH} variables were introduced, letting you suffer the penalties only on patterns marked with /p. Slay.
And from Perl 5.18.0 onwards, perl started noticing the presence of each of those three variables separately, and only copied the part of the string that was actually needed. So in:
$`; $&; "abcdefgh" =~ /d/perl would only copy the "abcd" part of the string. That could make a big difference in something like
$str = 'x' x 1_000_000;
$&; # whoops
$str =~ /x/g # one char copied a million times, not a million charsbestie. look at that. one tiny $& and suddenly your million-character string is getting copied a million times. it's giving tragedy. 😭
but ok good news!! in Perl 5.20.0 a whole new copy-on-write system got turned on by default, and it finally fixes most of the performance issues with these three variables, and makes them safe to use literally anywhere. we love a redemption arc. 👑
and if you're out here trying to hunt down where your code is being messy with these problematic match variables, the Devel::NYTProf and Devel::FindAmpersand modules can help you find uses of them. your besties for the investigation era. 🕵️♀️
$<digits> ($1, $2, ...)
contains the subpattern from the corresponding set of capturing parentheses from the last successful pattern match in the current dynamic scope. (see "Scoping Rules of Regex Variables".)
ok so important lil distinction: there is a difference between a capture buffer that matches the empty string, and a capture buffer that is <em>optional</em>. like, (x?) and (x)? are NOT the same vibe. the latter may be undef, the former will not be. know the difference, it matters. ✨
these variables are read-only.
mnemonic: like \digits.
@{^CAPTURE}
an array that exposes the contents of the capture buffers, if any, of the last successful pattern match — <em>not</em> counting patterns matched in nested blocks that have already been exited.
heads up: the 0 index of @{^CAPTURE} is equivalent to $1, the 1 index is equivalent to $2, etc. (yes the indexing is offset, no it's not a bug, it's just how it is 🙈)
if ("foal"=~/(.)(.)(.)(.)/) {
print join "-", @{^CAPTURE};
}should output f-o-a-l. slay. 🐴
see also $<digits> ($1, $2, ...), %{^CAPTURE} and %{^CAPTURE_ALL}.
ngl, unlike most other regex magic variables, there is no single letter equivalent to @{^CAPTURE}. also be aware that when you're interpolating subscripts of this array you MUST use the demarcated variable form, for instance
print "${^CAPTURE[0]}"see "Demarcated variable names using braces" in perldata for more info on this form and its uses.
this variable was added in 5.25.7.
and if you need access to this functionality in older Perls you can use this function immediately after your regexp:
sub get_captures {
no strict 'refs';
my $last_idx = scalar(@-) - 1;
my @arr = 1 .. $last_idx;
my @ret = map { $$_; } @arr;
return @ret;
}$MATCH
$&
the string matched by the last successful pattern match. (see "Scoping Rules of Regex Variables".)
see "Performance issues" above for the serious performance implications of using this variable — even <em>once</em> — in your code. not me warning you twice, but like. consider yourself warned. 💀
this variable is read-only, and its value is dynamically scoped.
mnemonic: like & in some editors.
${^MATCH}
it is only guaranteed to return a defined value when the pattern was compiled or executed with the /p modifier.
this is similar to $& ($MATCH) except that to use it you must use the /p modifier when executing the pattern, and it does not incur the performance penalty associated with that variable.
see "Performance issues" above.
this variable was added in Perl v5.10.0.
this variable is read-only, and its value is dynamically scoped.
$PREMATCH
$`
the string preceding whatever was matched by the last successful pattern match. (see "Scoping Rules of Regex Variables").
see "Performance issues" above for the serious performance implications of using this variable — even once — in your code. yeah same energy. 😭
this variable is read-only, and its value is dynamically scoped.
mnemonic: ` often precedes a quoted string.
${^PREMATCH}
it is only guaranteed to return a defined value when the pattern was executed with the /p modifier.
this is similar to $` ($PREMATCH) except that to use it you must use the /p modifier when executing the pattern, and it does not incur the performance penalty associated with that variable.
see "Performance issues" above.
this variable was added in Perl v5.10.0.
this variable is read-only, and its value is dynamically scoped.
$POSTMATCH
$'
the string following whatever was matched by the last successful pattern match. (see "Scoping Rules of Regex Variables"). example:
local $_ = 'abcdefghi';
/def/;
print "$`:$&:$'\n"; # prints abc:def:ghisee "Performance issues" above for the serious performance implications of using this variable — even once — in your code. (third time's the charm, bestie 🎀)
this variable is read-only, and its value is dynamically scoped.
mnemonic: ' often follows a quoted string.
${^POSTMATCH}
it is only guaranteed to return a defined value when the pattern was compiled or executed with the /p modifier.
this is similar to $' ($POSTMATCH) except that to use it you must use the /p modifier when executing the pattern, and it does not incur the performance penalty associated with that variable.
see "Performance issues" above.
this variable was added in Perl v5.10.0.
this variable is read-only, and its value is dynamically scoped.
$LAST_PAREN_MATCH
$+
the text matched by the highest used capture group of the last successful search pattern. (see "Scoping Rules of Regex Variables"). it is logically equivalent to the highest numbered capture variable ($1, $2, ...) which has a defined value.
this is SO useful if you don't know which one of a set of alternative patterns matched. for example:
/Version: (.*)|Revision: (.*)/ && ($rev = $+);this variable is read-only, and its value is dynamically scoped.
mnemonic: be positive and forward looking. 💖 (iconic advice honestly)
$LAST_SUBMATCH_RESULT
$^N
the text matched by the used group most-recently closed (i.e. the group with the rightmost closing parenthesis) of the last successful match. (see "Scoping Rules of Regex Variables").
this is <em>subtly</em> different from $+. for example in
"ab" =~ /^((.)(.))$/we have
$1,$^N have the value "ab"
$2 has the value "a"
$3,$+ have the value "b"this is primarily used inside (?{...}) blocks for examining text recently matched. for example, to effectively capture text to a variable (in addition to $1, $2, etc.), replace (...) with
(?:(...)(?{ $var = $^N }))by setting and then using $var in this way relieves you from having to worry about exactly which numbered set of parentheses they are. no more counting parens in your head, we're free 🦋
this variable is read-only, and its value is dynamically scoped.
this variable was added in Perl v5.8.0.
mnemonic: the (possibly) Nested parenthesis that most recently closed.
@LAST_MATCH_END
@+
this array holds the offsets of the ends of the last successful match and any matching capture buffers that the pattern contains. (see "Scoping Rules of Regex Variables")
the number of elements it contains will be one more than the number of capture buffers in the pattern, regardless of which capture buffers actually matched. you can use this to determine how many capture buffers there are in the pattern. (as opposed to @- which may have fewer elements.) 👀
$+[0] is the offset into the string of the end of the entire match. this is the same value as what the pos function returns when called on the variable that was matched against. the <em>n</em>th element of this array holds the offset of the <em>n</em>th submatch, so $+[1] is the offset past where $1 ends, $+[2] the offset past where $2 ends, and so on. you can use $#+ to determine how many subgroups were in the last successful match. see the examples given for the @- variable.
this variable is read-only, and its value is dynamically scoped.
this variable was added in Perl v5.6.0.
%{^CAPTURE}
%LAST_PAREN_MATCH
%+
similar to @+, the %+ hash allows access to the named capture buffers, should they exist, in the last successful match in the currently active dynamic scope. (see "Scoping Rules of Regex Variables").
for example, $+{foo} is equivalent to $1 after the following match:
'foo' =~ /(?<foo>foo)/;the keys of the %+ hash list only the names of buffers that have captured (and that are thus associated to defined values). so no ghost keys in here, only the ones that actually showed up. ✨
if multiple distinct capture groups have the same name, then $+{NAME} will refer to the leftmost defined group in the match.
the underlying behaviour of %+ is provided by the Tie::Hash::NamedCapture module.
Note:%-and%+are tied views into a <em>common internal hash</em> associated with the last successful regular expression. therefore mixing iterative access to them viaeachmay have unpredictable results. likewise, if the last successful match changes, then the results may be surprising. 👻
this variable was added in Perl v5.10.0. the %{^CAPTURE} alias was added in 5.25.7.
this variable is read-only, and its value is dynamically scoped.
@LAST_MATCH_START
@-
this array holds the offsets of the beginnings of the last successful match and any capture buffers it contains. (see "Scoping Rules of Regex Variables").
the number of elements it contains will be one more than the number of the highest capture buffers (also called a subgroup) that actually matched something. (as opposed to @+ which may have more elements.) it's giving "less is more" but for array lengths. 💅
$-[0] is the offset of the start of the last successful match. $-[n] is the offset of the start of the substring matched by <em>n</em>-th subpattern, or undef if the subpattern did not match.
thus, after a match against $_, $& coincides with substr $_, $-[0], $+[0] - $-[0]. similarly, $n coincides with substr $_, $-[n], $+[n] - $-[n] if $-[n] is defined, and $+ coincides with substr $_, $-[$#-], $+[$#-] - $-[$#-]. one can use $#- to find the last matched subgroup in the last successful match. contrast with $#+, the number of subgroups in the regular expression.
$-[0] is the offset into the string where the entire match starts. And the nth element of this array? That's the offset of the nth submatch 💅 so $-[1] is where $1 begins, $-[2] is where $2 begins, and so on forever and ever.
After a match against some variable $var, bestie, here's the whole cheat sheet:
"$`" is the same as "substr($var, 0, $-[0])"
$& is the same as "substr($var, $-[0], $+[0] - $-[0])"
"$'" is the same as "substr($var, $+[0])"
$1 is the same as "substr($var, $-[1], $+[1] - $-[1])"
$2 is the same as "substr($var, $-[2], $+[2] - $-[2])"
$3 is the same as "substr($var, $-[3], $+[3] - $-[3])"This variable is read-only, and its value is dynamically scoped. 🫶
It was added in Perl v5.6.0, so it's been iconic for a minute. ⭐
%{^CAPTURE_ALL}, %-
Basically %+'s cousin ✨ — this one gives you access to the named capture groups in the last successful match in the currently active dynamic scope. (See "Scoping Rules of Regex Variables"). For every capture group name chillin' in the regular expression, it hooks you up with a reference to an array holding the list of values captured by all buffers with that name (yes, there can be several of them 😭), in the order they appear. Obsessed.
Here's an example, bestie:
if ('1234' =~ /(?<A>1)(?<B>2)(?<A>3)(?<B>4)/) {
foreach my $bufname (sort keys %-) {
my $ary = $-{$bufname};
foreach my $idx (0..$#$ary) {
print "\$-{$bufname}[$idx] : ",
(defined($ary->[$idx])
? "'$ary->[$idx]'"
: "undef"),
"\n";
}
}
}and that would print out:
$-{A}[0] : '1'
$-{A}[1] : '3'
$-{B}[0] : '2'
$-{B}[1] : '4'The keys of the %- hash correspond to all buffer names found in the regular expression. Periodt.
The behaviour of %- is implemented via the Tie::Hash::NamedCapture module. 🧚
Note:%-and%+are tied views into a common internal hash associated with the last successful regular expression. So mixing iterative access to them viaeachmay have unpredictable results 💀 Likewise, if the last successful match changes, then the results may be surprising. See "Scoping Rules of Regex Variables".
This variable was added in Perl v5.10.0. The %{^CAPTURE_ALL} alias was added in 5.25.7.
This variable is read-only, and its value is dynamically scoped.
${^LAST_SUCCESSFUL_PATTERN}
The last successful pattern that matched in the current scope 👑 The empty pattern defaults to matching to this. For instance:
if (m/foo/ || m/bar/) {
s//BLAH/;
}and
if (m/foo/ || m/bar/) {
s/${^LAST_SUCCESSFUL_PATTERN}/BLAH/;
}are equivalent. Same energy, fr fr.
You can use this to debug which pattern matched last, or to match with it again. Lowkey a lifesaver when your regexes are giving chaos. 🦋
Added in Perl 5.37.10.
$LAST_REGEXP_CODE_RESULT, $^R
The result of evaluation of the last successful (?{ code }) regular expression assertion (see perlre). 🔥
This variable may be written to, and its value is scoped normally, unlike most other regex variables. Not like the others, main character energy. 💅
This variable was added in Perl 5.005.
${^RE_COMPILE_RECURSION_LIMIT}
The current value giving the maximum number of open but unclosed parenthetical groups there may be at any point during a regular expression compilation. The default is currently 1000 nested groups. You may adjust it depending on your needs and the amount of memory available. 🌸
This variable was added in Perl v5.30.0.
${^RE_DEBUG_FLAGS}
The current value of the regex debugging flags. Set it to 0 for no debug output even when the re 'debug' module is loaded. See re for details. ✨
This variable was added in Perl v5.10.0.
${^RE_TRIE_MAXBUF}
Controls how certain regex optimisations are applied and how much memory they utilize. This value by default is 65536 which corresponds to a 512kB temporary cache. Set this to a higher value to trade memory for speed when matching large alternations. Set it to a lower value if you want the optimisations to be as conservative of memory as possible but still occur, and set it to a negative value to prevent the optimisation and conserve the most memory. Under normal situations this variable should be of no interest to you. 🙈
This variable was added in Perl v5.10.0.
🌈 Variables related to filehandles 🌈
Variables that depend on the currently selected filehandle may be set by calling an appropriate object method on the IO::Handle object, although this is slightly less efficient than using the regular built-in variables. (Summary lines below for this contain the word HANDLE. 👀)
In Perl versions before v5.14.0, you had to say
use IO::Handle;before using
HANDLE->method(EXPR)Since Perl v5.14.0, you can use all filehandles as objects without manually loading any modules first. Glow up. ✨
Each method returns the old value of the IO::Handle attribute. The methods each take an optional EXPR, which, if supplied, specifies the new value for the IO::Handle attribute in question. If not supplied, most methods do nothing to the current value--except for autoflush(), which will assume a 1 for you, just to be different. 🙃
A few of these variables are considered "read-only". This means that if you try to assign to this variable, either directly or indirectly through a reference, you'll raise a run-time exception. 💀
You should be very careful when modifying the default values of most special variables described in this document. In most cases you want to localize these variables before changing them, since if you don't, the change may affect other modules which rely on the default values of the special variables that you have changed. This is one of the correct ways to read the whole file at once:
open my $fh, "<", "foo" or die $!;
local $/; # enable localized slurp mode
my $content = <$fh>;
close $fh;But the following code is quite bad:
open my $fh, "<", "foo" or die $!;
undef $/; # enable slurp mode
my $content = <$fh>;
close $fh;since some other module may want to read data from some file in the default "line mode", so if the code we have just presented has been executed, the global value of $/ is now changed for any other code running inside the same Perl interpreter. Not it. 😭
Usually when a variable is localized, you want to make sure that this change affects the shortest scope possible. So unless you are already inside some short {} block, you should create one yourself. For example:
my $content = '';
open my $fh, "<", "foo" or die $!;
{
local $/;
$content = <$fh>;
}
close $fh;Here is an example of how your own code can go broken:
for ( 1..3 ){
$\ = "\r\n";
nasty_break();
print "$_";
}
sub nasty_break {
$\ = "\f";
# do something with $_
}You probably expect this code to print the equivalent of
"1\r\n2\r\n3\r\n"but instead you get:
"1\f2\f3\f"Why? Because nasty_break() modifies $\ without localizing it first. The value you set in nasty_break() is still there when you return. The fix is to add local() so the value doesn't leak out of nasty_break():
local $\ = "\f";It's easy to notice the problem in such a short example, but in more complicated code you are looking for trouble if you don't localize changes to the special variables. Bestie, I'm begging you, localize. 🙏
$ARGV
Contains the name of the current file when reading from <>. 📖
@ARGV
The array @ARGV contains the command-line arguments intended for the script. $#ARGV is generally the number of arguments minus one, because $ARGV[0] is the first argument, not the program's command name itself. See $0 for the command name. 👑
ARGV
The special filehandle that iterates over command-line filenames in @ARGV. Usually written as the null filehandle in the diamond operator <>. Note that currently ARGV only has its magical effect within the <> operator; elsewhere it is just a plain filehandle corresponding to the last file opened by <>. In particular, passing \*ARGV as a parameter to a function that expects a filehandle may not cause your function to automatically read the contents of all the files in @ARGV. Careful out there. 💀
ARGVOUT
The special filehandle that points to the currently open output file when doing edit-in-place processing with -i. Useful when you have to do a lot of inserting and don't want to keep modifying $_. See perlrun for the -i switch. 🎀
IO::Handle->output_field_separator( EXPR ), $OUTPUT_FIELD_SEPARATOR, $OFS, $,
The output field separator for the print operator. If defined, this value is printed between each of print's arguments. Default is undef. ✨
You cannot call output_field_separator() on a handle, only as a static method. See IO::Handle.
Mnemonic: what is printed when there is a "," in your print statement. iykyk. 💅
HANDLE->input_line_number( EXPR ), $INPUT_LINE_NUMBER, $NR, $.
Current line number for the last filehandle accessed. 🔢
Each filehandle in Perl counts the number of lines that have been read from it. (Depending on the value of $/, Perl's idea of what constitutes a line may not match yours.) When a line is read from a filehandle (via readline() or <>), or when tell() or seek() is called on it, $. becomes an alias to the line counter for that filehandle. 🦋
You can adjust the counter by assigning to $., but this will not actually move the seek pointer. Localizing $. will not localize the filehandle's line count. Instead, it will localize perl's notion of which filehandle $. is currently aliased to. Sneaky little thing. 🙈
$. gets reset when the filehandle is closed… but NOT when an open filehandle gets reopened without an intervening close(). Wanna know more? Go read "I/O Operators" in perlop, bestie. And because <> never does an explicit close, your line numbers just keep climbing across ARGV files (but peep the examples in "eof" in perlfunc 👀).
You can also hit up HANDLE->input_line_number(EXPR) to grab the line counter for a specific filehandle without spiraling about which handle you last touched. Game changer, honestly.
Mnemonic: so many programs use "." to mean the current line number. Periodt.
🍓 the input record separator era 🍓
IO::Handle->input_record_separator( EXPR )
$INPUT_RECORD_SEPARATOR
$RS
$/$/ is the input record separator, newline by default. It's literally what decides Perl's idea of what a "line" even is. Works just like awk's RS variable — including treating empty lines as a terminator if you set it to the null string (and no, an empty line can't contain any spaces or tabs, it's giving strict). You may set it to a multi-character string to match a multi-character terminator, or to undef to read all the way through the end of file. 🌈
Setting it to "\n\n" means something slightly different than setting it to "", if the file has consecutive empty lines. Setting to "" will treat two or more consecutive empty lines as a single empty line. Setting to "\n\n" will blindly assume that the next input character belongs to the next paragraph, even if it's a newline. She's bold like that. 💅
local $/; # enable "slurp" mode
local $_ = <FH>; # whole file now here
s/\n[ \t]+/ /g;Remember: the value of $/ is a string, not a regex. awk has to be better for something. :-)
Setting $/ to an empty string — the so-called paragraph mode — merits special attention, fr fr. When $/ is set to "" and the entire file is read in with that setting, any sequence of one or more consecutive newlines at the beginning of the file is discarded. With the exception of the final record in the file, each sequence of characters ending in two or more newlines is treated as one record and is read in to end in exactly two newlines. If the last record in the file ends in zero or one consecutive newlines, that record is read in with that number of newlines. If the last record ends in two or more consecutive newlines, it is read in with two newlines like all preceding records. ✨
Suppose we wrote the following string to a file:
my $string = "\n\n\n";
$string .= "alpha beta\ngamma delta\n\n\n";
$string .= "epsilon zeta eta\n\n";
$string .= "theta\n";
my $file = 'simple_file.txt';
open my $OUT, '>', $file or die;
print $OUT $string;
close $OUT or die;Now we read that file in paragraph mode:
local $/ = ""; # paragraph mode
open my $IN, '<', $file or die;
my @records = <$IN>;
close $IN or die;@records will consist of these 3 strings:
(
"alpha beta\ngamma delta\n\n",
"epsilon zeta eta\n\n",
"theta\n",
)Setting $/ to a reference to an integer, scalar containing an integer, or scalar that's convertible to an integer will attempt to read records instead of lines, with the maximum record size being the referenced integer number of characters. So this:
local $/ = \32768; # or \"32768", or \$var_containing_32768
open my $fh, "<", $myfile or die $!;
local $_ = <$fh>;will read a record of no more than 32768 characters from $fh. If you're not reading from a record-oriented file (or your OS doesn't have record-oriented files), then you'll likely get a full chunk of data with every read. If a record is larger than the record size you've set, you'll get the record back in pieces. 🔪
Trying to set the record size to zero or less is deprecated and will cause$/to have the value ofundef, which will cause reading in the (rest of the) whole file. Not the vibe you wanted, bestie. 😭
As of 5.19.9, setting $/ to any other form of reference will throw a fatal exception. This is in preparation for supporting new ways to set $/ in the future. She's evolving, ok??
On VMS only, record reads bypass PerlIO layers and any associated buffering, so you must not mix record and non-record reads on the same filehandle. Record mode mixes with line mode only when the same buffering layer is in use for both modes. Respect the boundary 🫶
You cannot call input_record_separator() on a handle, only as a static method. See IO::Handle.
See also "Newlines" in perlport. Also see $..
Mnemonic: / delimits line boundaries when quoting poetry. 🎀
💖 output record separator 💖
IO::Handle->output_record_separator( EXPR )
$OUTPUT_RECORD_SEPARATOR
$ORS
$\$\ is the output record separator for the print operator. If defined, this value is printed after the last of print's arguments. Default is undef.
You cannot call output_record_separator() on a handle, only as a static method. See IO::Handle.
Mnemonic: you set $\ instead of adding \n at the end of the print. Also, it's just like $/, but it's what you get "back" from Perl. 🔁
🦋 autoflush szn 🦋
HANDLE->autoflush( EXPR )
$OUTPUT_AUTOFLUSH
$|If set to nonzero, forces a flush right away and after every write or print on the currently selected output channel. Default is 0 (regardless of whether the channel is really buffered by the system or not; $| tells you only whether you've asked Perl explicitly to flush after each write). STDOUT will typically be line buffered if output is to the terminal and block buffered otherwise. 👑
Setting this variable is useful primarily when you are outputting to a pipe or socket, such as when you are running a Perl program under rsh and want to see the output as it's happening. This has no effect on input buffering. See "getc" in perlfunc for that. See "select" in perlfunc on how to select the output channel. See also IO::Handle.
Mnemonic: when you want your pipes to be piping hot. 🔥
⭐ ${^LAST_FH} ⭐
This read-only variable contains a reference to the last-read filehandle. This is set by <HANDLE>, readline, tell, eof and seek. This is the same handle that $. and tell and eof without arguments use. It is also the handle used when Perl appends ", <STDIN> line 1" to an error or warning message.
This variable was added in Perl v5.18.0. ✨
🎀 Variables related to formats 🎀
The special variables for formats are a subset of those for filehandles. See perlform for more information about Perl's formats.
$ACCUMULATOR
$^A$^A is the current value of the write() accumulator for format() lines. A format contains formline() calls that put their result into $^A. After calling its format, write() prints out the contents of $^A and empties. So you never really see the contents of $^A unless you call formline() yourself and then look at it. See perlform and "formline PICTURE,LIST" in perlfunc.
IO::Handle->format_formfeed(EXPR)
$FORMAT_FORMFEED
$^L$^L is what formats output as a form feed. The default is "\f".
You cannot call format_formfeed() on a handle, only as a static method. See IO::Handle.
HANDLE->format_page_number(EXPR)
$FORMAT_PAGE_NUMBER
$%$% is the current page number of the currently selected output channel.
Mnemonic: "%" is page number in nroff.
HANDLE->format_lines_left(EXPR)
$FORMAT_LINES_LEFT
$-$- is the number of lines left on the page of the currently selected output channel.
Mnemonic: lines_on_page - lines_printed.
IO::Handle->format_line_break_characters EXPR
$FORMAT_LINE_BREAK_CHARACTERS
$:$: is the current set of characters after which a string may be broken to fill continuation fields (starting with "^") in a format. The default is " \n-", to break on a space, newline, or a hyphen.
You cannot call format_line_break_characters() on a handle, only as a static method. See IO::Handle.
Mnemonic: a "colon" in poetry is a part of a line. 🌸
HANDLE->format_lines_per_page(EXPR)
$FORMAT_LINES_PER_PAGE
$=$= is the current page length (printable lines) of the currently selected output channel. The default is 60.
Mnemonic: = has horizontal lines.
HANDLE->format_top_name(EXPR)
$FORMAT_TOP_NAME
$^$^ is the name of the current top-of-page format for the currently selected output channel. The default is the name of the filehandle with "_TOP" appended. For example, the default format top name for the "STDOUT" filehandle is "STDOUT_TOP".
Mnemonic: points to top of page.
HANDLE->format_name(EXPR)
$FORMAT_NAME
$~$~ is the name of the current report format for the currently selected output channel. The default format name is the same as the filehandle name. For example, the default format name for the "STDOUT" filehandle is just "STDOUT".
Mnemonic: brother to $^.
😭 Error Variables 😭
The variables $@, $!, $^E, and $? contain information about different types of error conditions that may appear during execution of a Perl program. The variables are shown ordered by the "distance" between the subsystem which reported the error and the Perl process. They correspond to errors detected by the Perl interpreter, C library, operating system, or an external program, respectively. 📏
To illustrate the differences between these variables, consider the following Perl expression, which uses a single-quoted string. After execution of this statement, perl may have set all four special error variables:
eval q{
open my $pipe, "/cdrom/install |" or die $!;
my @res = <$pipe>;
close $pipe or die "bad pipe: $?, $!";
};When perl executes the eval() expression, it translates the open(), <PIPE>, and close calls in the C run-time library and thence to the operating system kernel. perl sets $! to the C library's "errno" if one of these calls fails. 💀
$@ gets set if the string you "eval"-ed didn't compile (which can happen if open or close got imported with bad prototypes, oops), or if Perl code running during that evaluation went and die()d. In those cases $@ holds either the compile error or whatever got passed to "die" (and yeah, that argument will interpolate $! and $?). (See also Fatal, though. 🫶)
On a few operating systems, $^E might carry a way more chatty error indicator, like literally "CDROM tray not closed." Systems that don't do extended error messages just leave $^E identical to $!.
And finally, $? can get set to a non-0 value if that external program /cdrom/install fails. The upper eight bits reflect the specific error conditions the program hit (its exit() value). The lower eight bits reflect the mode of failure, like signal death and core dump info. See wait(2) for the details. And here's the glow-up difference: $! and $^E only get set when an error condition is detected, but $? gets set on every "wait" or pipe "close", overwriting whatever was there before. So it's way more like $@, which on every eval() is always set on failure and cleared on success. 💅
Want more? The individual descriptions at $@, $!, $^E, and $? have you covered.
${^CHILD_ERROR_NATIVE} 🍼
The native status returned by the last pipe close, backtick (``) command, successful call to wait() or waitpid(), or from the system() operator. On POSIX-like systems you can decode this value with the WIFEXITED, WEXITSTATUS, WIFSIGNALED, WTERMSIG, WIFSTOPPED, and WSTOPSIG functions that the POSIX module hands you. ✨
Under VMS this reflects the actual VMS exit status — i.e. it's the same as $? when the pragma use vmsish 'status' is in effect.
This variable was added in Perl v5.10.0.
$EXTENDED_OS_ERROR, $^E 💥
Error information specific to the current operating system. Right now, this only differs from "$!" under VMS, OS/2, and Win32 (and for MacPerl). On literally every other platform, $^E is always just the same as $!.
- Under VMS,
$^Egives you the VMS status value from the last system error. That's more specific info about the last system error than$!gives you. Which is especially important when$!is set toEVMSERR. - Under OS/2,
$^Eis set to the error code of the last call to the OS/2 API, whether that came via CRT or straight from perl. - Under Win32,
$^Ealways returns the last error info reported by the Win32 callGetLastError(), which describes the last error from inside the Win32 API. Most Win32-specific code reports errors via$^E. Meanwhile ANSI C and Unix-like calls set"errno", so most portable Perl code reports errors via$!.
The caveats mentioned in the description of "$!" generally apply to $^E too, bestie. 🙈
This variable was added in Perl 5.003.
Mnemonic: Extra error explanation.
$EXCEPTIONS_BEING_CAUGHT, $^S 🎭
Current state of the interpreter.
$^S State
--------- -------------------------------------
undef Parsing module, eval, or main program
true (1) Executing an eval or try block
false (0) OtherwiseThat first state can show up inside $SIG{__DIE__} and $SIG{__WARN__} handlers.
Heads up: the English name $EXCEPTIONS_BEING_CAUGHT is slightly misleading, because the "undef" value does not tell you whether exceptions are being caught — compilation of the main program doesn't catch exceptions, periodt.
This variable was added in Perl 5.004.
$WARNING, $^W ⚠️
The current value of the warning switch. It starts out true if -w was used, false otherwise, but you can modify it directly. Go off.
See also warnings.
Mnemonic: related to the -w switch.
${^WARNING_BITS} 🚨
The current set of warning checks enabled by the "use warnings" pragma. It has the same scoping as the $^H and "%^H" variables. The exact values are considered internal to the warnings pragma and may change between versions of Perl. So like, don't get attached. 😭
Every time a statement finishes being compiled, the current value of "${^WARNING_BITS}" is stored with that statement, and can later be retrieved via "(caller($level))[9]".
This variable was added in Perl v5.6.0.
$OS_ERROR, $ERRNO, $! 🔥
When you reference it, $! retrieves the current value of the C "errno" integer variable. If you assign $! a numerical value, that value gets stored in "errno". And when you reference it as a string, $! yields the system error string corresponding to "errno".
Lots of system or library calls set "errno" when they fail, to show you why they failed. But they usually don't set "errno" to zero when they succeed, and they might set "errno" to a non-zero value on success. Which means "errno", and therefore $!, is only meaningful immediately after a failure:
if (open my $fh, "<", $filename) {
# Here $! is meaningless.
...
}
else {
# ONLY here is $! meaningful.
...
# Already here $! might be meaningless.
}
# Since here we might have either success or failure,
# $! is meaningless.Here, meaningless means $! may have nothing to do with how that open() actually went. Assigning to $! is just as ephemeral, bestie. You can use it right before invoking the die() operator, to set the exit value, or to inspect the system error string corresponding to error n, or to restore $! to a meaningful state. 🫶
Perl itself may set "errno" to a non-zero value on failure even if no system call is performed.
Mnemonic: What just went bang? 💀
%OS_ERROR, %ERRNO, %! 🗺️
Each element of "%!" has a true value only if $! is set to that value. For example, $!{ENOENT} is true if and only if the current value of $! is "ENOENT" — that is, if the most recent error was "No such file or directory" (or its moral equivalent: not every operating system hands you that exact error, and certainly not every language does). The specific true value isn't guaranteed, but in the past it's generally been the numeric value of $!. Wanna check if a particular key is even meaningful on your system? Use "exists $!{the_key}". Wanna see the legal keys? Use "keys %!". See Errno for more, and also see "$!".
This variable was added in Perl 5.005.
$CHILD_ERROR, $? 👶
The status returned by the last pipe close, backtick (``) command, successful call to wait() or waitpid(), or from the system() operator. This is literally the 16-bit status word the traditional Unix wait() system call returns (or else it's made up to look like it). So the exit value of the subprocess is really ($? >> 8), and "$? & 127" gives you which signal, if any, the process died from, and "$? & 128" reports whether there was a core dump. 🌈
Also, if the "h_errno" variable is supported in C, its value gets returned via $? if any "gethost*()" function fails.
If you've installed a signal handler for "SIGCHLD", the value of $? will usually be wrong outside that handler. Noted. 😬
Inside an "END" subroutine, $? contains the value that's about to be given to exit(). And yes, you can modify $? in an "END" subroutine to change your program's exit status. Like:
END {
$? = 1 if $? == 255; # die would make it 255
}Under VMS, the pragma use vmsish 'status' makes $? reflect the actual VMS exit status instead of the default emulation of POSIX status; see "$?" in perlvms for the details.
Mnemonic: similar to sh and ksh.
$EVAL_ERROR, $@ 💌
The Perl error from the last "eval" operator, i.e. the last exception that got caught. For "eval BLOCK", this is either a runtime error message or the string or reference die was called with. The "eval STRING" form also catches syntax errors and other compile time exceptions. Iconic, honestly.
If no error occurs, "eval" sets $@ to the empty string.
Warning messages are not collected in this variable. You can, however, set up a routine to process warnings by setting $SIG{__WARN__} as described in "%SIG".
Mnemonic: Where was the error "at"?
🧠 Variables related to the interpreter state 🧠
These variables serve up info about the current interpreter state. Vibe check your own runtime, bestie.
$COMPILING, $^C 🔨
The current value of the flag associated with the -c switch. Mainly useful with -MO=... to let code alter its behavior while it's being compiled — like, for example, to "AUTOLOAD" at compile time instead of doing the normal, deferred loading thing. Setting "$^C = 1" is similar to calling "B::minus_c".
This variable was added in Perl v5.6.0.
$DEBUGGING, $^D 🐛
The current value of the debugging flags. You can read it or set it. Just like its command-line equivalent, you can use numeric or symbolic values, e.g. "$^D = 10" or "$^D = "st"". See "-Dnumber" in perlrun. The contents of this variable also affect how the debugger operates. See "Debugger Internals" in perldebguts.
Mnemonic: value of -D switch.
${^GLOBAL_PHASE} 🌀
The current phase of the perl interpreter.
Possible values are:
CONSTRUCT— The"PerlInterpreter*"is being constructed via"perl_construct". This value is mostly there for completeness and for use via the underlying C variable"PL_phase". It's not really possible for Perl code to be executed unless construction of the interpreter is finished.START— This is the global compile-time. That includes, basically, every"BEGIN"block executed directly or indirectly from during the compile-time of the top-level program.
okay so this phase is not called BEGIN to avoid confusion with BEGIN-blocks, because those run during compile-time of any compilation unit, not just the top-level program. A new, localised compile-time entered at run-time — for example by constructs like eval "use SomeModule" — are not global interpreter phases, so they aren't reflected by ${^GLOBAL_PHASE}. 💅
CHECK— Execution of anyCHECKblocks. ✨INIT— Similar toCHECK, but forINIT-blocks, notCHECKblocks. 🎀RUN— The main run-time, i.e. the execution ofPL_main_root. 💖END— Execution of anyENDblocks. 🌸DESTRUCT— Global destruction. 💀
Also note: there's no value for UNITCHECK-blocks. That's because those are run for each compilation unit individually, so it's not a global interpreter phase. 🫶
Not every program has to go through each of the possible phases, but transition from one phase to another can only happen in the order described in the above list. Periodt. 👑
An example of all of the phases Perl code can see:
BEGIN { print "compile-time: ${^GLOBAL_PHASE}\n" }
INIT { print "init-time: ${^GLOBAL_PHASE}\n" }
CHECK { print "check-time: ${^GLOBAL_PHASE}\n" }
{
package Print::Phase;
sub new {
my ($class, $time) = @_;
return bless \$time, $class;
}
sub DESTROY {
my $self = shift;
print "$$self: ${^GLOBAL_PHASE}\n";
}
}
print "run-time: ${^GLOBAL_PHASE}\n";
my $runtime = Print::Phase->new(
"lexical variables are garbage collected before END"
);
END { print "end-time: ${^GLOBAL_PHASE}\n" }
our $destruct = Print::Phase->new(
"package variables are garbage collected after END"
);This will print out
compile-time: START
check-time: CHECK
init-time: INIT
run-time: RUN
lexical variables are garbage collected before END: RUN
end-time: END
package variables are garbage collected after END: DESTRUCTThis variable was added in Perl 5.14.0. 🦋
✨ $^H ✨
WARNING: This variable is strictly for internal use only. Its availability, behavior, and contents are subject to change without notice. 💀
This variable contains compile-time hints for the Perl interpreter. At the end of compilation of a BLOCK the value of this variable is restored to the value when the interpreter started to compile the BLOCK. 🧚
Each time a statement completes being compiled, the current value of $^H is stored with that statement, and can later be retrieved via (caller($level))[8]. See "caller EXPR" in perlfunc. 📞
When perl begins to parse any block construct that provides a lexical scope (e.g., eval body, required file, subroutine body, loop body, or conditional block), the existing value of $^H is saved, but its value is left unchanged. When the compilation of the block is completed, it regains the saved value. Between the points where its value is saved and restored, code that executes within BEGIN blocks is free to change the value of $^H. 💅
This behavior provides the semantic of lexical scoping, and is used in, for instance, the use strict pragma. 👑
The contents should be an integer; different bits of it are used for different pragmatic flags. Here's an example:
sub add_100 { $^H |= 0x100 }
sub foo {
BEGIN { add_100() }
bar->baz($boon);
}Consider what happens during execution of the BEGIN block. At this point the BEGIN block has already been compiled, but the body of foo() is still being compiled. The new value of $^H will therefore be visible only while the body of foo() is being compiled. 🤔
Substitution of BEGIN { add_100() } block with:
BEGIN { require strict; strict->import('vars') }demonstrates how use strict 'vars' is implemented. Here's a conditional version of the same lexical pragma:
BEGIN {
require strict; strict->import('vars') if $condition
}This variable was added in Perl 5.003. 🎀
💜 %^H 💜
The %^H hash provides the same scoping semantics as $^H. This makes it useful for implementing lexically scoped pragmas. See perlpragma. All the entries are stringified when accessed at runtime, so only simple values can be accommodated. This means no references to objects, for example. 🫶
Each time a statement completes being compiled, the current value of %^H is stored with that statement, and can later be retrieved via (caller($level))[10]. See "caller EXPR" in perlfunc. 📞
When putting items into %^H, in order to avoid conflicting with other users of the hash there is a convention regarding which keys to use. A module should use only keys that begin with the module's name (the name of its main package) and a / character. For example, a module Foo::Bar should use keys such as Foo::Bar/baz. 🔑
This variable was added in Perl v5.6.0. ✨
🌈 ${^OPEN} 🌈
An internal variable used by PerlIO. A string in two parts, separated by a "\0" byte, the first part describes the input layers, the second part describes the output layers. 💖
This is the mechanism that applies the lexical effects of the open pragma, and the main program scope effects of the "io" or "D" options for the -C command-line switch and PERL_UNICODE environment variable. 🌟
The functions accept(), open(), pipe(), readpipe() (as well as the related "qx" and `STRING` operators), socket(), socketpair(), and sysopen() are affected by the lexical value of this variable. The implicit "ARGV" handle opened by readline() (or the related "<>" (diamond) and "<<>>" (double diamond) operators) on passed filenames is also affected (but not if it opens "STDIN"). If this variable is not set, these functions will set the default layers as described in "Defaults and how to override them" in PerlIO. 🎀
open() ignores this variable (and the default layers) when called with 3 arguments and explicit layers are specified. Indirect calls to these functions via modules like IO::Handle are not affected as they occur in a different lexical scope. Directory handles such as opened by opendir() are not currently affected. 🚫
This variable was added in Perl v5.8.0. 💅
🔥 $PERLDB, $^P 🔥
The internal variable for debugging support. The meanings of the various bits are subject to change, but currently indicate:
0x01— Debug subroutine enter/exit. 🐛0x02— Line-by-line debugging. CausesDB::DB()subroutine to be called for each statement executed. Also causes saving source code lines (like0x400). 📝0x04— Switch off optimizations. 🚫0x08— Preserve more data for future interactive inspections. 🔍0x10— Keep info about source lines on which a subroutine is defined. 📍0x20— Start with single-step on. 👣0x40— Use subroutine address instead of name when reporting. 🏷️0x80— Report"goto &subroutine"as well. 🏃0x100— Provide informative"file"names for evals based on the place they were compiled. 📂0x200— Provide informative names to anonymous subroutines based on the place they were compiled. 🏷️0x400— Save source code lines into@{"_<$filename"}. 💾0x800— When saving source, include evals that generate no subroutines. 🧬0x1000— When saving source, include source that did not compile. 🧟
Some bits may be relevant at compile-time only, some at run-time only. This is a new mechanism and the details may change. See also perldebguts. 👀
💀 ${^TAINT} 💀
Reflects if taint mode is on or off. 1 for on (the program was run with -T), 0 for off, -1 when only taint warnings are enabled (i.e. with -t or -TU). 🚨
Note: if your perl was built without taint support (see perlsec), then ${^TAINT} will always be 0, even if the program was run with -T). 😭
This variable is read-only. 🔒
This variable was added in Perl v5.8.0. ✨
🌍 ${^SAFE_LOCALES} 🌍
Reflects if safe locale operations are available to this perl (when the value is 1) or not (the value is 0). This variable is always 1 if the perl has been compiled without threads. It is also 1 if this perl is using thread-safe locale operations. Note that an individual thread may choose to use the global locale (generally unsafe) by calling "switch_to_global_locale" in perlapi. This variable currently is still set to 1 in such threads. 🧵
This variable is read-only. 🔒
This variable was added in Perl v5.28.0. 🎉
🌐 ${^UNICODE} 🌐
Reflects certain Unicode settings of Perl. See perlrun documentation for the -C switch for more information about the possible values. 🦄
This variable is set during Perl startup and is thereafter read-only. 🔒
This variable was added in Perl v5.8.2. ✨
🧚 ${^UTF8CACHE} 🧚
This variable controls the state of the internal UTF-8 offset caching code. 1 for on (the default), 0 for off, -1 to debug the caching code by checking all its results against linear scans, and panicking on any discrepancy. 😱
This variable was added in Perl v5.8.9. It is subject to change or removal without notice, but is currently used to avoid recalculating the boundaries of multi-byte UTF-8-encoded characters. 🔥
${^UTF8LOCALE}— okay so this one is a little vibe check 💅 it tells you whether perl clocked a UTF-8 locale when it started up. Perl uses this info when it's in adjust-utf8ness-to-locale mode (like when you run it with the-CLcommand-line switch). Wanna know more? Go peek at perlrun, bestie. ✨ Added in Perl v5.8.8.
💀 Deprecated and removed variables 💀
Deprecating a variable is basically the perl maintainers saying "bestie, we're gonna remove this eventually, don't get too attached." It might still be around, but using it throws a warning. Once it's actually removed, using it triggers an error that's like "unsupported, sorry not sorry." For the deets on error messages, check perldiag. 🫶
$#— this one used to format printed numbers. After a whole deprecation cycle, its magic got yeeted in Perl v5.10.0, and now using it gives you a compilation error:"$# is no longer supported". 🚫 Not to be confused with the sigil you put in front of an array name to get the last index, like$#array. That's still very much a thing in Perl — it's how you get the last index of an array. (See "Arrays" in perlintro.) The two have literally nothing to do with each other, fr fr. Deprecated in Perl 5. Removed in Perl v5.10.0.$*— this one used to let you enable multiline matching. After a deprecation cycle, its magic was removed in Perl v5.10.0. Using it now triggers a compilation error:"$* is no longer supported". Instead, you should use the/sand/mregexp modifiers. ✨ Deprecated in Perl 5. Removed in Perl v5.10.0.$[— this stores the index of the first element in an array, and the first character in a substring. Default is0, but you could theoretically set it to1to make Perl act more like awk (or Fortran) when subscripting and when evaluatingindex()andsubstr(). As of release 5 of Perl, assignment to$[is treated as a compiler directive, and can't influence the behavior of any other file. (That's why you can only assign compile-time constants to it.) Its use is highly discouraged, bestie, like don't. 🚩 Prior to Perl v5.10.0, assignment to$[could be seen from outer lexical scopes in the same file, unlike other compile-time directives (likestrict). Usinglocal()on it would bind its value strictly to a lexical block. Now it's always lexically scoped. As of Perl v5.16.0, it's implemented by thearybasemodule. As of Perl v5.30.0, or underuse v5.16, orno feature "array_base",$[no longer has any effect, and always contains0. Assigning0to it is permitted, but any other value will produce an error. Mnemonic:[begins subscripts. Deprecated in Perl v5.12.0.${^ENCODING}— this variable is no longer supported. It used to hold the object reference to theEncodeobject that was used to convert source code to Unicode. Its purpose was to let your non-ASCII Perl scripts not have to be written in UTF-8; that was useful before editors that worked on UTF-8 encoded text were common, but that was ages ago. It caused problems, like affecting the operation of other modules that weren't expecting it, causing general mayhem. 😭 If you need something like this functionality, it's recommended you use a simple source filter, such asFilter::Encoding. If you're here because code of yours is being adversely affected by someone's use of this variable, you can usually work around it by doing this:
local ${^ENCODING};near the beginning of the functions that are getting broken. This undefines the variable during the scope of execution of the including function. This variable was added in Perl 5.8.2 and removed in 5.26.0. Setting it to anything other than undef was made fatal in Perl 5.28.0. 💀
${^WIN32_SLOPPY_STAT}— this variable no longer has any function, periodt. Added in Perl v5.10.0 and removed in Perl v5.34.0. 🫶
And that's a wrap on PERLVAR(1), bestie! This page is from perl v5.42.3, dated 2026-08-03. Stay iconic, keep your variables close, and don't use deprecated ones. 💖✨
📦 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 : a language model's rewrite of the original, under its terms