PERLRUN(1perl)

💖 PERLRUN(1perl) 💖

✨ NAME ✨

perlrun — how to execute the Perl interpreter 🫶

🌈 SYNOPSIS 🌈

perl [ -gsTtuUWX ]      [ -h?v ] [ -V[:configvar] ]
     [ -cw ] [ -d[t][:debugger] ] [ -D[number/list] ]
     [ -pna ] [ -Fpattern ] [ -l[octal] ] [ -0[octal/hexadecimal] ]
     [ -Idir ] [ -m[-]module ] [ -M[-]'module...' ] [ -f ]
     [ -C [number/list] ]      [ -S ]      [ -x[dir] ]
     [ -i[extension] ]
     [ [-e|-E] 'command' ] [ -- ] [ programfile ] [ argument ]...

🌸 DESCRIPTION 🌸

okay so the normal way to run a Perl program is by making it directly executable, or else by passing the name of the source file as an argument on the command line. (An interactive Perl environment is also possible — see perldebug for details on how to do that.) Upon startup, Perl looks for your program in one of the following places:

1. Specified line by line via -e or -E switches on the command line.
2. Contained in the file specified by the first filename on the command line. (Note that systems supporting the #! notation invoke interpreters this way. See "Location of Perl".)
3. Passed in implicitly via standard input. This works only if there are no filename arguments — to pass arguments to a STDIN-read program you must explicitly specify a "-" for the program name.

With methods 2 and 3, Perl starts parsing the input file from the beginning, unless you've specified a -x switch, in which case it scans for the first line starting with #! and containing the word perl, and starts there instead. This is useful for running a program embedded in a larger message. (In this case you would indicate the end of the program using the __END__ token.)

The #! line is always examined for switches as the line is being parsed. Thus, if you're on a machine that allows only one argument with the #! line, or worse, doesn't even recognize the #! line, you still can get consistent switch behaviour regardless of how Perl was invoked, even if -x was used to find the beginning of the program.

Because historically some operating systems silently chopped off kernel interpretation of the #! line after 32 characters, some switches may be passed in on the command line, and some may not; you could even get a - without its letter, if you're not careful. You probably want to make sure that all your switches fall either before or after that 32-character boundary. Most switches don't actually care if they're processed redundantly, but getting a - instead of a complete switch could cause Perl to try to execute standard input instead of your program. And a partial -I switch could also cause odd results.

Some switches do care if they are processed twice, for instance combinations of -l and -0. Either put all the switches after the 32-character boundary (if applicable), or replace the use of -0digits by BEGIN{ $/ = "\0digits"; }.

Parsing of the #! switches starts wherever perl is mentioned in the line. The sequences -* and - are specifically ignored so that you could, if you were so inclined, say

#!/bin/sh
#! -*- perl -*- -p
eval 'exec perl -x -wS $0 ${1+"$@"}'
    if 0;

to let Perl see the -p switch.

A similar trick involves the env program, if you have it.

#!/usr/bin/env perl

The examples above use a relative path to the perl interpreter, getting whatever version is first in the user's path. If you want a specific version of Perl, say, perl5.14.1, you should place that directly in the #! line's path.

If the #! line does not contain the word perl nor the word indir, the program named after the #! is executed instead of the Perl interpreter. This is slightly bizarre, but it helps people on machines that don't do #!, because they can tell a program that their SHELL is /usr/bin/perl, and Perl will then dispatch the program to the correct interpreter for them.

After locating your program, Perl compiles the entire program to an internal form. If there are any compilation errors, execution of the program is not attempted. (This is unlike the typical shell script, which might run part-way through before finding a syntax error.)

If the program is syntactically correct, it is executed. If the program runs off the end without hitting an exit() or die() operator, an implicit exit(0) is provided to indicate successful completion.

🦋 #! and quoting on non-Unix systems 🦋

Unix's #! technique can be simulated on other systems:

OS/2

Put

extproc perl -S -your_switches

as the first line in *.cmd file (-S due to a bug in cmd.exe's extproc handling).

MS-DOS

Create a batch file to run your program, and codify it in ALTERNATE_SHEBANG (see the dosish.h file in the source distribution for more information).

Win95/NT

The Win95/NT installation, when using the ActiveState installer for Perl, will modify the Registry to associate the .pl extension with the perl interpreter. If you install Perl by other means (including building from the sources), you may have to modify the Registry yourself. Note that this means you can no longer tell the difference between an executable Perl program and a Perl library file.

VMS

Put

$ perl -mysw 'f$env("procedure")' 'p1' 'p2' 'p3' 'p4' 'p5' 'p6' 'p7' 'p8' !
$ exit++ + ++$status != 0 and $exit = $status = undef;

at the top of your program, where -mysw are any command line switches you want to pass to Perl. You can now invoke the program directly, by saying perl program, or as a DCL procedure, by saying @program (or implicitly via DCL$PATH by just using the name of the program).

This incantation is a bit much to remember, but Perl will display it for you if you say perl "-V:startperl".

Command-interpreters on non-Unix systems have rather different ideas on quoting than Unix shells. You'll need to learn the special characters in your command-interpreter (*, \ and " are common) and how to protect whitespace and these characters to run one-liners (see -e below).

On some systems, you may have to change single-quotes to double ones, which you must not do on Unix or Plan 9 systems. You might also have to change a single % to a %%.

For example:

# Unix
perl -e 'print "Hello world\n"'

# MS-DOS, etc.
perl -e "print \"Hello world\n\""

# VMS
perl -e "print ""Hello world\n"""

The problem is that none of this is reliable: it depends on the command and it is entirely possible neither works. If 4DOS were the command shell, this would probably work better:

perl -e "print <Ctrl-x>"Hello world\n<Ctrl-x>""

CMD.EXE in Windows NT slipped a lot of standard Unix functionality in when nobody was looking, but just try to find documentation for its quoting rules.

There is no general solution to all of this. It's just a mess. 😭

🌟 Location of Perl 🌟

It may seem obvious to say, but Perl is useful only when users can easily find it. When possible, it's good for both /usr/bin/perl and /usr/local/bin/perl to be symlinks to the actual binary. If that can't be done, system administrators are strongly encouraged to put (symlinks to) perl and its accompanying utilities into a directory typically found along a user's PATH, or in some other obvious and convenient place.

In this documentation, #!/usr/bin/perl on the first line of the program will stand in for whatever method works on your system. You are advised to use a specific path if you care about a specific version.

#!/usr/local/bin/perl5.14

or if you just want to be running at least version, place a statement like this at the top of your program:

use v5.14;

👑 Command Switches 👑

As with all standard commands, a single-character switch may be clustered with the following switch, if any.

#!/usr/bin/perl -spi.orig   # same as -s -p -i.orig

A -- signals the end of options and disables further option processing. Any arguments after the -- are treated as filenames and arguments.

Switches include:

• -0[octal/hexadecimal] — specifies the input record separator ($/) as an octal or hexadecimal number. If there are no digits, the null character is the separator. Other switches may precede or follow the digits. For example, if you have a version of find which can print filenames terminated by the null character, you can say this:

find . -name '*.orig' -print0 | perl -n0e unlink

The special value 00 will cause Perl to slurp files in paragraph mode.

Any value 0400 or above will cause Perl to slurp files whole, but by convention the value 0777 is the one normally used for this purpose. The -g flag is a simpler alias for it.

You can also specify the separator character using hexadecimal notation: -0xHHH..., where the H are valid hexadecimal digits. Unlike the octal form, this one may be used to specify any Unicode character, even those beyond 0xFF. So if you really want a record separator of 0777, specify it as -0x1FF. (This means that you cannot use the -x option with a directory name that consists of hexadecimal digits, or else Perl will think you have specified a hex number to -0.)

• -a — turns on autosplit mode when used with a -n or -p. An implicit split command to the @F array is done as the first thing inside the implicit while loop produced by the -n or -p.

perl -ane 'print pop(@F), "\n";'

is equivalent to

while (<>) {
    @F = split(' ');
    print pop(@F), "\n";
}

An alternate delimiter may be specified using -F.

-a implicitly sets -n.

• -C [number/list] — The -C flag controls some of the Perl Unicode features.

CAUTION: As with the :utf8 PerlIO layer, none of the features enabled by this flag or the equivalent PERL_UNICODE environment variable validate that input is valid UTF-8, nor guarantee to produce valid UTF-8. Instead it will assume input is provided in Perl's internal upgraded byte encoding, and provide output in this encoding, which is a superset of UTF-8 that can encode any character allowed in Perl strings. (On EBCDIC systems, it is a superset of UTF-EBCDIC instead.) This can result in broken Perl strings or output bytes which are not valid in UTF-8. This internal encoding will be referred to as "utf8" below to differentiate it from a strict UTF-8 encoding format.

okay so as of 5.8.1, -C can be followed by either a number or a whole list of option letters ✨ and listing the letters is literally the same thing as summing those numbers, bestie. here's the tea:

    I     1   STDIN is assumed to be in utf8
    O     2   STDOUT will be in utf8
    E     4   STDERR will be in utf8
    S     7   I + O + E
    i     8   :utf8 is the default PerlIO layer for input streams
    o    16   :utf8 is the default PerlIO layer for output streams
    D    24   i + o
    A    32   the @ARGV elements are expected to be strings encoded
              in utf8
    L    64   normally the "IOEioA" are unconditional, the L makes
              them conditional on the locale environment variables
              (the LC_ALL, LC_CTYPE, and LANG, in the order of
              decreasing precedence) -- if the variables indicate
              UTF-8, then the selected "IOEioA" are in effect
    a   256   Set ${^UTF8CACHE} to -1, to run the UTF-8 caching
              code in debugging mode.

so like, -COE and -C6 both turn on utf8-ness for both STDOUT and STDERR 🎀 they're giving the same energy. and repeating letters? just redundant — not cumulative, not toggling. periodt.

the io options mean that any subsequent open() (or similar I/O operations) in main program scope will have the :utf8 PerlIO layer implicitly applied to them 💅 in other words, utf8 is expected from any input stream, and utf8 is produced to any output stream. this is just the default set via ${^OPEN}, with explicit layers in open() and with binmode() you can manipulate streams as usual. this has no effect on code run in modules.

-C on its own (not followed by any number or option list), or the empty string "" for the PERL_UNICODE environment variable, has the same effect as -CSDL 🌈 in other words, the standard I/O handles and the default open() layer are utf8-fied but only if the locale environment variables indicate a UTF-8 locale. this behaviour follows the implicit (and problematic) UTF-8 behaviour of Perl 5.8.0. (see "UTF-8 no longer default under UTF-8 locales" in perl581delta.)

you can use -C0 (or "0" for PERL_UNICODE) to explicitly disable all the above Unicode features 🚫

the read-only magic variable ${^UNICODE} reflects the numeric value of this setting. this variable is set during Perl startup and is thereafter read-only. want runtime effects instead? use the three-arg open() (see "open" in perlfunc), the two-arg binmode() (see "binmode" in perlfunc), and the open pragma (see open).

(in Perls earlier than 5.8.1 the -C switch was a Win32-only switch that enabled the use of Unicode-aware "wide system call" Win32 APIs. this feature was practically unused, however, and the command line switch was therefore "recycled".)

Note: Since perl 5.10.1, if the -C option is used on the #! line, it must be specified on the command line as well, since the standard streams are already set up at this point in the execution of the perl interpreter. You can also use binmode() to set the encoding of an I/O stream.

---

• -c — makes Perl check the syntax of the program and then dip out without executing it. but actually — plot twist — it will execute any BEGIN, UNITCHECK, or CHECK blocks and any use statements, because those are considered as occurring outside the execution of your program. INIT and END blocks, however, will be skipped 🙈 if the syntax check is successful perl will exit with a status of zero and report yourprogram syntax OK. on failure perl will print any detected errors and exit with a non-zero status. no cap.
• -d, -dt — runs the program under the Perl debugger 🐛 see perldebug. if t is specified, PERL5DB_THREADED=1 is set in the environment to tell the debugger that threads will be used in the code being debugged.
• -d:MOD[=bar,baz], -dt:MOD[=bar,baz] — runs the program under the control of a debugging, profiling, or tracing module installed as Devel::MOD ✨ e.g., -d:DProf executes the program using the Devel::DProf profiler. as with the -M flag, options may be passed to the Devel::MOD package where they'll be received and interpreted by the Devel::MOD::import routine. again, like -M, use --d:-MOD to call Devel::MOD::unimport instead of import. the comma-separated list of options must follow a = character. if t is specified, it indicates to the debugger that threads will be used in the code being debugged. see perldebug.
• -Dletters, -Dnumber — sets debugging flags. ngl this switch is enabled only if your perl binary has been built with debugging enabled: normal production perls won't have been.

for example, to watch how perl executes your program, use -Dtls. another nice value is -Dx, which lists your compiled syntax tree, and -Dr displays compiled regular expressions 🦋 the format of the output is explained in perldebguts.

as an alternative, specify a number instead of a list of letters (e.g., -D14 is equivalent to -Dtls):

            1  p  Tokenizing and parsing (with v, displays parse
                  stack)
            2  s  Stack snapshots (with v, displays all stacks)
            4  l  Context (loop) stack processing
            8  t  Trace execution
           16  o  Method and overloading resolution
           32  c  String/numeric conversions
           64  P  Print profiling info, source file input state
          128  m  Memory and SV allocation
          256  f  Format processing
          512  r  Regular expression parsing and execution
         1024  x  Syntax tree dump
         2048  u  Tainting checks
         4096  U  Unofficial, User hacking (reserved for private,
                  unreleased use)
         8192  h  Show hash randomization debug output (changes to
                  PL_hash_rand_bits and their origin)
        16384  X  Scratchpad allocation
        32768  D  Cleaning up
        65536  S  Op slab allocation
       131072  T  Tokenizing
       262144  R  Include reference counts of dumped variables
                  (eg when using -Ds)
       524288  J  show s,t,P-debug (don't Jump over) on opcodes within
                  package DB
      1048576  v  Verbose: use in conjunction with other flags to
                  increase the verbosity of the output.  Is a no-op on
                  many of the other flags
      2097152  C  Copy On Write
      4194304  A  Consistency checks on internal structures
      8388608  q  quiet - currently only suppresses the "EXECUTING"
                  message
     16777216  M  trace smart match resolution
     33554432  B  dump suBroutine definitions, including special
                  Blocks like BEGIN
     67108864  L  trace Locale-related info; what gets output is very
                  subject to change
    134217728  i  trace PerlIO layer processing.  Set PERLIO_DEBUG to
                  the filename to trace to.
    268435456  y  trace y///, tr/// compilation and execution

all these flags require -DDEBUGGING when you compile the Perl executable 👑 (but see ":opd" in Devel::Peek or "'debug' mode" in re which may change this). see the INSTALL file in the Perl source distribution for how to do this.

and bestie, if you're just trying to get a print out of each line of Perl code as it executes — the way that sh -x provides for shell scripts — you literally can't use Perl's -D switch. instead do this:

  # If you have "env" utility
  env PERLDB_OPTS="NonStop=1 AutoTrace=1 frame=2" perl -dS program

  # Bourne shell syntax
  $ PERLDB_OPTS="NonStop=1 AutoTrace=1 frame=2" perl -dS program

  # csh syntax
  % (setenv PERLDB_OPTS "NonStop=1 AutoTrace=1 frame=2"; perl -dS program)

see perldebug for details and variations.

• -e commandline — may be used to enter one line of program 💖 if -e is given, Perl will not look for a filename in the argument list. multiple -e commands may be given to build up a multi-line script. make sure to use semicolons where you would in a normal program, okay?? sloppy semicolons are not the vibe.
• -E commandline — behaves just like -e, except that it implicitly enables all optional features and builtin functions (in the main compilation unit) ✨ see feature and builtin.
• -f — disables executing $Config{sitelib}/sitecustomize.pl at startup.

Perl can be built so that it by default will try to execute $Config{sitelib}/sitecustomize.pl at startup (in a BEGIN block). this is a hook that lets the sysadmin customize how Perl behaves 🎀 it can for instance be used to add entries to the @INC array to make Perl find modules in non-standard locations.

Perl actually inserts the following code:

  BEGIN {
      do { local $!; -f "$Config{sitelib}/sitecustomize.pl"; }
          && do "$Config{sitelib}/sitecustomize.pl";
  }

since it is an actual do (not a require), sitecustomize.pl doesn't need to return a true value. the code is run in package main, in its own lexical scope. however, if the script dies, $@ will not be set 😭

the value of $Config{sitelib} is also determined in C code and not read from Config.pm, which is not loaded.

the code is executed very early. for example, any changes made to @INC will show up in the output of perl -V. of course, END blocks will be likewise executed very late.

to determine at runtime if this capability has been compiled in your perl, you can check the value of $Config{usesitecustomize}.

• -Fpattern — specifies the pattern to split on for -a 🔪 the pattern may be surrounded by //, "", or '', otherwise it will be put in single quotes. you can't use literal whitespace or NUL characters in the pattern, bestie, so don't even try.

-F implicitly sets both -a and -n 🫶

• -g — undefines the input record separator ($/) and thus enables the slurp mode 🍓 in other words, it causes Perl to read whole files at once, instead of line by line.

this flag is a simpler alias for -0777.

mnemonics: gobble, grab, gulp 👑

• -h — prints a summary of the options. iconic. helpful. we stan a cheat sheet.
• -? — synonym for -h: prints a summary of the options 💅 same energy, same output.

---

-i[extension] — the in-place edit glow-up 💅

okay bestie, this is THE one. -i[extension] says: every file that gets processed by the "<>" construct is gonna get edited in-place. Periodt.

How does it pull that off? It renames the input file, opens the output file using the original name, and then selects that output file as the default for print() statements. The extension, if you supplied one, is what tweaks the old file's name to make a backup copy, following these rules:

• If you supply no extension, and your system supports it, the original file stays open without a name while the output gets redirected to a brand new file with the original filename. When perl exits — cleanly or not — that original file is unlinked. 💀
• If the extension doesn't contain a *, then it just gets appended to the end of the current filename as a suffix.
• If the extension does contain one or more * characters, then each * is replaced with the current filename. In Perl brain terms, think of it as:

($backup = $extension) =~ s/\*/$file_name/g;

This is the way you add a prefix to the backup file, instead of (or on top of) a suffix:

 $ perl -pi'orig_*' -e 's/bar/baz/' fileA  # backup to
                                           # 'orig_fileA'

Or lowkey place backup copies of the original files into a whole other directory (as long as that directory already exists, okay?):

 $ perl -pi'old/*.orig' -e 's/bar/baz/' fileA  # backup to
                                               # 'old/fileA.orig'

These sets of one-liners are literally equivalent:

 $ perl -pi -e 's/bar/baz/' fileA          # overwrite current file
 $ perl -pi'*' -e 's/bar/baz/' fileA       # overwrite current file

 $ perl -pi'.orig' -e 's/bar/baz/' fileA   # backup to 'fileA.orig'
 $ perl -pi'*.orig' -e 's/bar/baz/' fileA  # backup to 'fileA.orig'

So from the shell, saying

    $ perl -p -i.orig -e "s/foo/bar/; ... "

is the same as using the program:

    #!/usr/bin/perl -pi.orig
    s/foo/bar/;

which is equivalent to

#!/usr/bin/perl
$extension = '.orig';
LINE: while (<>) {
    if ($ARGV ne $oldargv) {
        if ($extension !~ /\*/) {
            $backup = $ARGV . $extension;
        }
        else {
            ($backup = $extension) =~ s/\*/$ARGV/g;
        }
        rename($ARGV, $backup);
        open(ARGVOUT, ">$ARGV");
        select(ARGVOUT);
        $oldargv = $ARGV;
    }
    s/foo/bar/;
}
continue {
    print;  # this prints to original filename
}
select(STDOUT);

...except that the -i form doesn't need to compare $ARGV to $oldargv to know when the filename has changed. It does, however, use ARGVOUT for the selected filehandle. And note, ✨vibe check✨: STDOUT is restored as the default output filehandle after the loop. 🫶

As shown above, Perl creates the backup file whether or not any output is actually changed. So honestly this is just a fancy way to copy files:

    $ perl -p -i'/some/file/path/*' -e 1 file1 file2 file3...

or

    $ perl -p -i'.orig' -e 1 file1 file2 file3...

You can use "eof" without parentheses to locate the end of each input file, in case you wanna append to each file, or reset line numbering (see the example in "eof" in perlfunc).

If, for a given file, Perl is unable to create the backup file as specified in the extension, then it will skip that file and continue on with the next one (if it exists, bestie).

For a whole discussion of issues surrounding file permissions and -i, go read "Why does Perl let me delete read-only files? Why does -i clobber protected files? Isn't this a bug in Perl?" in perlfaq5.

You cannot use -i to create directories or to strip extensions from files. That's just not its job, periodt. 🙅‍♀️

Perl does not expand "~" in filenames, which is good actually, since some folks use it for their backup files:

    $ perl -pi~ -e 's/foo/bar/' file1 file2 file3...

Note that because -i renames or deletes the original file before creating a new file of the same name, Unix-style soft and hard links will not be preserved. 😭 devastating, I know.

Finally, the -i switch does not impede execution when no files are given on the command line. In this case, no backup is made (the original file cannot, of course, be determined) and processing proceeds from STDIN to STDOUT as might be expected. 🌈

---

-Idirectory

Directories specified by -I are prepended to the search path for modules (@INC). It's giving priority access ✨

---

-l[octnum]

enables automatic line-ending processing. It has two separate effects, and both matter:

1. It automatically chomps $/ (the input record separator) when used with "-n" or "-p".
2. It assigns "$\" (the output record separator) to have the value of octnum, so that any print statements will have that separator added back on.

If octnum is omitted, it sets "$\" to the current value of $/. For instance, to trim lines to 80 columns:

    perl -lpe 'substr($_, 80) = ""'

Note that the assignment "$\ = $/" is done when the switch is processed, so the input record separator can be different than the output record separator if the -l switch is followed by a -0 switch:

    gnufind / -print0 | perl -ln0e 'print "found $_" if -p'

This sets "$\" to newline and then sets $/ to the null character. Two for the price of one, iconic. 💅

---

-m[-]module

-M[-]module

-M[-]'module ...'

-[mM][-]module=arg[,arg]...

-mmodule executes "use module ();" before executing your program. This loads the module, but does not call its "import" method, so it does not import subroutines and does not give effect to a pragma.

-Mmodule executes "use module;" before executing your program. This loads the module and calls its "import" method, causing the module to have its default effect, typically importing subroutines or giving effect to a pragma. You can use quotes to add extra code after the module name, e.g., '-MMODULE qw(foo bar)'.

If the first character after the -M or -m is a dash (-) then the 'use' is replaced with 'no'. This makes no difference for -m.

Since Perl version 5.39.8, the "-M" switch and the module name can now be passed in separate command-line arguments:

  perl -M MODULE ...            # like perl -MMODULE ...
  perl -M MODULE=arg1,arg2,...  # like perl -MMODULE=arg1,arg2,...

A little builtin syntactic sugar means you can also say -mMODULE=foo,bar or -MMODULE=foo,bar as a shortcut for '-MMODULE qw(foo bar)'. This avoids the need to use quotes when importing symbols. The actual code generated by -MMODULE=foo,bar is "use module split(/,/,q{foo,bar})". Note that the "=" form removes the distinction between -m and -M; that is, -mMODULE=foo,bar is the same as -MMODULE=foo,bar.

A consequence of the "split" formulation is that -MMODULE=number never does a version check, unless "MODULE::import()" itself is set up to do a version check, which could happen for example if MODULE inherits from Exporter. 👑

---

-n

causes Perl to assume the following loop around your program, which makes it iterate over filename arguments somewhat like sed -n or awk:

  LINE:
    while (<>) {
        ...             # your program goes here
    }

Note that the lines are not printed by default. See "-p" to have lines printed. If a file named by an argument cannot be opened for some reason, Perl warns you about it and moves on to the next file.

Also note that "<>" passes command line arguments to "open" in perlfunc, which doesn't necessarily interpret them as file names. See perlop for possible security implications. 🙈

Here is an efficient way to delete all files that haven't been modified for at least a week:

    find . -mtime +7 -print | perl -nle unlink

This is faster than using the -exec switch of find because you don't have to start a process on every filename found (but it's not faster than using the -delete switch available in newer versions of find. It does suffer from the bug of mishandling newlines in pathnames, which you can fix if you follow the example under -0.

"BEGIN" and "END" blocks may be used to capture control before or after the implicit program loop, just as in awk.

---

-p

causes Perl to assume the following loop around your program, which makes it iterate over filename arguments somewhat like sed:

  LINE:
    while (<>) {
        ...             # your program goes here
    } continue {
        print or die "-p destination: $!\n";
    }

If a file named by an argument cannot be opened for some reason, Perl warns you about it, and moves on to the next file. Note that the lines are printed automatically. An error occurring during printing is treated as fatal. To suppress printing use the "-n" switch. A -p overrides a -n switch.

"BEGIN" and "END" blocks may be used to capture control before or after the implicit loop, just as in awk.

---

-s

enables rudimentary switch parsing for switches on the command line after the program name but before any filename arguments (or before an argument of --). Any switch found there is removed from @ARGV and sets the corresponding variable in the Perl program, in the main package. The following program prints "1" if the program is invoked with a -xyz switch, and "abc" if it is invoked with -xyz=abc.

    #!/usr/bin/perl -s
    if ($xyz) { print "$xyz\n" }

Do note that a switch like --help creates the variable "${-help}", which is not compliant with "use strict "refs"". Also, when using this option on a script with warnings enabled you may get a lot of spurious "used only once" warnings. For these reasons, use of -s is discouraged. See Getopt::Long for much more flexible switch parsing. 🫣

---

-S

makes Perl use the "PATH" environment variable to search for the program unless the name of the program contains path separators.

On some platforms, this also makes Perl append suffixes to the filename while it's out here searching, bestie. Like, on Win32 it'll tack on the .bat and .cmd suffixes if the lookup for the original name fails, and if the name doesn't already end in one of those suffixes. And if your Perl was compiled with DEBUGGING turned on, you can throw the -Dp switch at Perl and it'll show you, step by step, how the search is going. 🕵️‍♀️

Typically this is used to emulate #! startup on platforms that don't support #!. It's also convenient when debugging a script that uses #!, and is thus normally found by the shell's $PATH search mechanism.

This example works on many platforms that have a shell compatible with Bourne shell:

#!/usr/bin/perl
eval 'exec /usr/bin/perl -wS $0 ${1+"$@"}'
        if 0; # ^ Run only under a shell

The system ignores the first line and feeds the program to /bin/sh, which proceeds to try to execute the Perl program as a shell script. The shell executes the second line as a normal shell command, and thus starts up the Perl interpreter. On some systems $0 doesn't always contain the full pathname, so the -S tells Perl to search for the program if necessary. After Perl locates the program, it parses the lines and ignores them because the check if 0 is never true. If the program will be interpreted by csh, you will need to replace ${1+"$@"} with $*, even though that doesn't understand embedded spaces (and such) in the argument list. To start up sh rather than csh, some systems may have to replace the #! line with a line containing just a colon, which will be politely ignored by Perl. Other systems can't control that, and need a totally devious construct that will work under any of csh, sh, or Perl, such as the following:

        eval '(exit $?0)' && eval 'exec perl -wS $0 ${1+"$@"}'
        & eval 'exec /usr/bin/perl -wS $0 $argv:q'
                if 0; # ^ Run only under a shell

If the filename supplied contains directory separators (and so is an absolute or relative pathname), and if that file is not found, platforms that append file extensions will do so and try to look for the file with those extensions added, one by one.

On DOS-like platforms, if the program does not contain directory separators, it will first be searched for in the current directory before being searched for on the PATH. On Unix platforms, the program will be searched for strictly on the PATH. know the difference, it matters ✨

---

• -t — Like -T, but taint checks will issue warnings rather than fatal errors. These warnings can now be controlled normally with no warnings qw(taint).

Note: This is not a substitute for -T! This is meant to be used only as a temporary development aid while securing legacy code: for real production code and for new secure code written from scratch, always use the real -T. fr fr, don't play with this one 🙈

This has no effect if your perl was built without taint support.

• -T — turns on "taint" so you can test them. Ordinarily these checks are done only when running setuid or setgid. It's a good idea to turn them on explicitly for programs that run on behalf of someone else whom you might not necessarily trust, such as CGI programs or any internet servers you might write in Perl. See perlsec for details. For security reasons, this option must be seen by Perl quite early; usually this means it must appear early on the command line or in the #! line for systems which support that construct. main character energy: show up first or don't show up at all 💅
• -u — This switch causes Perl to dump core after compiling your program. You can then in theory take this core dump and turn it into an executable file by using the undump program (not supplied). This speeds startup at the expense of some disk space (which you can minimize by stripping the executable). (Still, a "hello world" executable comes out to about 200K on my machine.) If you want to execute a portion of your program before dumping, use the CORE::dump() function instead. Note: availability of undump is platform specific and may not be available for a specific port of Perl.
• -U — allows Perl to do unsafe operations. Currently the only "unsafe" operations are attempting to unlink directories while running as superuser and running setuid programs with fatal taint checks turned into warnings. Note that warnings must be enabled along with this option to actually generate the taint-check warnings.
• -v — prints the version and patchlevel of your perl executable. 📌
• -V — prints summary of the major perl configuration values and the current values of @INC.
• -V:configvar — Prints to STDOUT the value of the named configuration variable(s), with multiples when your configvar argument looks like a regex (has non-letters). For example:

$ perl -V:libc
    libc='/lib/libc-2.2.4.so';
$ perl -V:lib.
    libs='-lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc';
    libc='/lib/libc-2.2.4.so';
$ perl -V:lib.*
    libpth='/usr/local/lib /lib /usr/lib';
    libs='-lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc';
    lib_ext='.a';
    libc='/lib/libc-2.2.4.so';
    libperl='libperl.a';
    ....

Additionally, extra colons can be used to control formatting. A trailing colon suppresses the linefeed and terminator ;, allowing you to embed queries into shell commands. (mnemonic: PATH separator :.)

$ echo "compression-vars: " `perl -V:z.*: ` " are here !"
compression-vars:  zcat='' zip='zip'  are here !

A leading colon removes the name= part of the response, this allows you to map to the name you need. (mnemonic: empty label)

$ echo "goodvfork="`./perl -Ilib -V::usevfork`
goodvfork=false;

Leading and trailing colons can be used together if you need positional parameter values without the names. Note that in the case below, the PERL_API params are returned in alphabetical order.

$ echo building_on `perl -V::osname: -V::PERL_API_.*:` now
building_on 'linux' '5' '1' '9' now

• -w — prints warnings about dubious constructs, such as variable names mentioned only once and scalar variables used before being set; redefined subroutines; references to undefined filehandles; filehandles opened read-only that you are attempting to write on; values used as a number that don't look like numbers; using an array as though it were a scalar; if your subroutines recurse more than 100 deep; and innumerable other things.

This switch really just enables the global $^W variable; normally, the lexically scoped use warnings pragma is preferred. You can disable or promote into fatal errors specific warnings using __WARN__ hooks, as described in perlvar and warn in perlfunc. See also perldiag and perltrap. A fine-grained warning facility is also available if you want to manipulate entire classes of warnings; see warnings.

• -W — Enables all warnings regardless of no warnings or $^W. See warnings.
• -X — Disables all warnings regardless of use warnings or $^W. See warnings.

Forbidden in PERL5OPT. 🚫

• -x
• -xdirectory — tells Perl that the program is embedded in a larger chunk of unrelated text, such as in a mail message. Leading garbage will be discarded until the first line that starts with #! and contains the string perl. Any meaningful switches on that line will be applied.

All references to line numbers by the program (warnings, errors, ...) will treat the #! line as the first line. Thus a warning on the 2nd line of the program, which is on the 100th line in the file will be reported as line 2, not as line 100. This can be overridden by using the #line directive. (See "Plain Old Comments (Not!)" in perlsyn)

If a directory name is specified, Perl will switch to that directory before running the program. The -x switch controls only the disposal of leading garbage. The program must be terminated with __END__ if there is trailing garbage to be ignored; the program can process any or all of the trailing garbage via the DATA filehandle if desired.

The directory, if specified, must appear immediately following the -x with no intervening whitespace. no spaces. none. zero. ngl this gets people every time 😭

---

🌍 ENVIRONMENT 🌍

• HOME — Used if chdir has no argument.
• LOGDIR — Used if chdir has no argument and HOME is not set.
• PATH — Used in executing subprocesses, and in finding the program if -S is used.
• PERL5LIB — A list of directories in which to look for Perl library files before looking in the standard library. Any architecture-specific and version-specific directories, such as version/archname/, version/, or archname/ under the specified locations are automatically included if they exist, with this lookup done at interpreter startup time. In addition, any directories matching the entries in $Config{inc_version_list} are added. (These typically would be for older compatible perl versions installed in the same directory tree.)

If PERL5LIB is not defined, PERLLIB is used. Directories are separated (like in PATH) by a colon on Unixish platforms and by a semicolon on Windows (the proper path separator being given by the command perl -V:path_sep).

When running taint checks, either because the program was running setuid or setgid, or the -T or -t switch was specified, neither PERL5LIB nor PERLLIB is consulted. The program should instead say:

use lib "/my/directory";

okay that's the env stuff, keep scrolling bestie we're not done ✨

PERL5OPT 💖

Okay so imagine wanting certain switches to just show up on literally every Perl command line you ever run, without you typing them. That's PERL5OPT, bestie. Whatever switches you put in here get treated as if they were on every perl command line. Iconic.

But the guest list is strict — only the -[CDIMTUWdmtw] switches are allowed. That's it. No plus-ones.

🚨 The minute taint checks are happening — whether the program was running setuid or setgid, or the -T or -t switch was used — this whole variable gets ignored. Like, completely.

Two lil edge cases so you're not caught off guard:

• If PERL5OPT starts with -T, tainting gets enabled and the options after it get ignored.
• If PERL5OPT starts with -t, tainting gets enabled, a writable dot gets yoinked out of @INC, and the subsequent options are honored.

PERLIO 🌸

A space-separated (or colon-separated) list of PerlIO layers. If perl is built to use the PerlIO system for IO — which is the default — these layers affect how Perl does IO. Vibe check on your filehandles, basically.

So the convention is to start layer names with a colon, like :perlio, to make them look all fancy and similar to variable "attributes". But plot twist: the code that parses layer specification strings — the same code that decodes the PERLIO environment variable — treats that colon as a separator. So it's giving attribute, but it's actually a divider. 🙈

If PERLIO is unset or empty, that's equivalent to the default set of layers for your platform. For example, :unix:perlio on Unix-like systems, and :unix:crlf on Windows and other DOS-like systems.

This list becomes the default for all of Perl's IO. Which means only built-in layers can appear here — external layers (like :encoding()) need IO in order to load themselves, so they can't be in the starting lineup. See "open pragma" if you want to add external encodings as defaults.

Here are the layers that actually make sense to include in the PERLIO environment variable, briefly summarized. For more details see PerlIO.

• :crlf — a layer that does CRLF-to-"\n" translation, distinguishing "text" and "binary" files the way MS-DOS and similar operating systems do. On these architectures it also provides buffering similar to :perlio.
• :perlio — a re-implementation of stdio-like buffering, written as a PerlIO layer. Because of that, it calls whatever layer is below it for its operations, typically :unix.
• :stdio — gives you a PerlIO interface by wrapping the system's ANSI C stdio library calls. Provides both buffering and IO. Heads up though: :stdio does not do CRLF translation, even if that's the platform's normal behaviour. You'll need a :crlf layer sitting above it to pull that off.
• :unix — the low-level layer that calls read, write, lseek, etc.

The default set of layers should give acceptable results on all platforms.

For Unix platforms, that'll be the equivalent of :unix:perlio or :stdio. Configure is set up to prefer the :stdio implementation if the system's library provides for fast access to the buffer (which is not common on modern architectures); otherwise, it uses the :unix:perlio implementation.

On Win32, the default in this release (5.30) is :unix:crlf. Win32's :stdio has a number of bugs/mis-features for Perl IO, and they're somewhat dependent on the version and vendor of the C compiler. Using our own :crlf layer as the buffer dodges all those issues and keeps things more uniform. This release (5.30) uses :unix as the bottom layer on Win32, so it still uses the C compiler's numeric file descriptor routines.

⚠️ The PERLIO environment variable is completely ignored when Perl is run in taint mode. Not "partially". Completely.

PERLIO_DEBUG 🎀

Set this to the name of a file or device while running Perl with the -Di command-line switch, and the logging of certain PerlIO subsystem operations gets redirected to that file instead of going to stderr (which is the default). The file is opened in append mode.

Typical use on Unix:

% env PERLIO_DEBUG=/tmp/perlio.log perl -Di script ...

And under Win32, the approximately-equivalent move:

> set PERLIO_DEBUG=CON
perl -Di script ...

🚫 This functionality is disabled for setuid scripts, for scripts run with -T, and for scripts running on a Perl built without -DDEBUGGING support.

PERLLIB 📚

A list of directories to look in for Perl library files before looking in the standard library. If PERL5LIB is defined, then PERLLIB doesn't get used. Sorry girl, one of you has to sit this one out.

⚠️ The PERLLIB environment variable is completely ignored when Perl is run in taint mode.

PERL5DB 🐛

The command used to load the debugger code. The default is:

BEGIN { require "perl5db.pl" }

💡 The PERL5DB environment variable is only used when Perl is started with a bare -d switch. Bare. No extras.

PERL5DB_THREADED 🧵

Set it to a true value and you're telling the debugger that the code being debugged uses threads. That's the whole job.

PERL5SHELL (specific to the Win32 port) 🪟

Win32 ports only, bestie. This one may be set to an alternative shell that Perl has to use internally for executing backtick commands or system(). Default is cmd.exe /x/d/c on WindowsNT and command.com /c on Windows95. The value is considered space-separated. And if a character needs protecting — like a space or a backslash — you precede it with another backslash.

Now, why not just use COMSPEC for this? Because COMSPEC has a high degree of variability among users, which leads to portability concerns. Also, Perl can use a shell that may not be fit for interactive use, and setting COMSPEC to that kind of shell may interfere with the proper functioning of other programs — which usually look in COMSPEC to find a shell that is fit for interactive use. So yeah, separate variable, separate business.

⚠️ Before Perl 5.10.0 and 5.8.8, PERL5SHELL was not taint checked when running external commands. It's recommended that you explicitly set (or delete) $ENV{PERL5SHELL} when running in taint mode under Windows. Do not skip this one.

PERL_ALLOW_NON_IFS_LSP (specific to the Win32 port) 🪟

Set this to 1 to allow the use of non-IFS compatible LSPs (Layered Service Providers). Normally Perl searches for an IFS-compatible LSP, because that's required for its emulation of Windows sockets as real filehandles. But that can cause problems if you've got a firewall like McAfee Guardian, which requires that all applications use its LSP — except it's not IFS-compatible. And obviously Perl will normally avoid using such an LSP. Awkward standoff.

Setting this environment variable to 1 means Perl will simply use the first suitable LSP enumerated in the catalog, which keeps McAfee Guardian happy. And in that particular case, Perl still works too, because McAfee Guardian's LSP actually plays other games which allow applications requiring IFS compatibility to work. Everybody wins. 💅

PERL_DEBUG_MSTATS 🧮

Only relevant if Perl is compiled with the malloc included with the Perl distribution — that is, if perl -V:d_mymalloc is define.

If it's set, this dumps out memory statistics after execution. And if you set it to an integer greater than one, it also dumps out memory statistics after compilation. Two for one.

PERL_DESTRUCT_LEVEL 💀

Controls the behaviour of global destruction of objects and other references. See PERL_DESTRUCT_LEVEL in perlhacktips for more info — that's where the real details live.

PERL_DL_NONLAZY 🔗

Set to 1 and Perl will resolve all undefined symbols when it loads a dynamic library. The default behaviour is to resolve symbols when they're used instead.

Setting this variable is genuinely useful when you're testing extensions, because it guarantees you get an error on misspelled function names even if the test suite never actually calls them. Catch those typos early, king. 👑

PERL_ENCODING 🔤

If you're using the use encoding pragma without an explicit encoding name, then the PERL_ENCODING environment variable gets consulted for an encoding name. It's the fallback bestie.

PERL_HASH_SEED 🎲

(Since Perl 5.8.1, with new semantics in Perl 5.18.0.) Used to override the randomization of Perl's internal hash function. The value is expressed in hexadecimal, and it may include a leading 0x. Truncated patterns get treated as though they're suffixed with sufficient 0's as required.

Now here's the switch-up: if the option is provided, and PERL_PERTURB_KEYS is NOT set, then a value of '0' implies PERL_PERTURB_KEYS=0 / PERL_PERTURB_KEYS=NO, and any other value implies PERL_PERTURB_KEYS=2 / PERL_PERTURB_KEYS=DETERMINISTIC. Go read the docs for PERL_PERTURB_KEYS for the important caveats about that DETERMINISTIC mode before you go messing with it.

🚨 PLEASE NOTE: the hash seed is sensitive information. Hashes are randomized specifically to protect against local and remote attacks against Perl code. By manually setting a seed, that protection may be partially or completely lost. This is not a cute little tweak, okay?

See "Algorithmic Complexity Attacks" in perlsec, plus PERL_PERTURB_KEYS and PERL_HASH_SEED_DEBUG, for more information.

PERL_PERTURB_KEYS 🌀

(Since Perl 5.18.0.) Set it to 0 or NO and traversing keys will be repeatable from run to run for the same PERL_HASH_SEED. Insertion into a hash won't change the order — except to provide for more space in the hash. When you combine this with setting PERL_HASH_SEED, this mode is as close to pre-5.18 behavior as you can possibly get. Time travel, but make it hashes. ⭐

• "1" or "RANDOM" — traversing keys gets fully randomized 🎲. Every time a hash is inserted into, the key order changes in a random fashion. The order may not be repeatable in a following program run, even if PERL_HASH_SEED has been specified. This is the default mode for perl when no PERL_HASH_SEED has been explicitly provided. 💅
• "2" or "DETERMINISTIC" — inserting keys into a hash will cause the key order to change, but in a way that's repeatable from program run to program run, provided that the same hash seed is used, and that the code doesn't itself perform any non-deterministic operations, and also provided exactly the same environment context. Adding or removing an environment variable may and likely will change the key order 😭. If any part of the code builds a hash using non-deterministic keys — for instance a hash keyed by the stringified form of a reference, or the address of the objects it contains — then this may and likely will have a global effect on the key order of every hash in the process 💀. To work properly this setting MUST be coupled with the PERL_HASH_SEED to produce deterministic results, and in fact, if you do set the PERL_HASH_SEED explicitly you do not need to set this as well — it will be automatically set to this mode. ✨

🚨 NOTE: use of this option is considered insecure, and is intended only for debugging non-deterministic behavior in Perl's hash function. Do not use it in production. no cap.

See "Algorithmic Complexity Attacks" in perlsec and PERL_HASH_SEED and PERL_HASH_SEED_DEBUG for more information. You can get and set the key traversal mask for a specific hash by using the hash_traversal_mask() function from Hash::Util. 🫶

---

💖 PERL_HASH_SEED_DEBUG

(Since Perl 5.8.1.) Set it to "1" and it displays (to STDERR) info about the hash function, the seed, and what type of key traversal randomization is in effect at the beginning of execution 👀. This, combined with PERL_HASH_SEED and PERL_PERTURB_KEYS, is intended to aid in debugging nondeterministic behaviour caused by hash randomization.

Note that any information about the hash function, especially the hash seed, is sensitive information: by knowing it, one can craft a denial-of-service attack against Perl code, even remotely; see "Algorithmic Complexity Attacks" in perlsec for more information. Do not disclose the hash seed to people who don't need to know it 🙈. See also hash_seed() and hash_traversal_mask().

An example output might be:

HASH_FUNCTION = ONE_AT_A_TIME_HARD HASH_SEED = 0x652e9b9349a7a032 PERTURB_KEYS = 1 (RANDOM)

---

🌈 PERL_MEM_LOG

If your Perl was configured with -Accflags=-DPERL_MEM_LOG, setting the environment variable PERL_MEM_LOG enables logging debug messages 🔍. The value has the form <number>[m][s][t], where number is the file descriptor number you want to write to (2 is default), and the combination of letters specifies what you want information about — (m)emory and/or (s)v — optionally with (t)imestamps ⏰. For example, PERL_MEM_LOG=1mst logs all information to stdout. You can write to other opened file descriptors in a variety of ways:

$ 3>foo3 PERL_MEM_LOG=3m perl ...

---

🦋 PERL_ROOT (VMS port only, bestie)

A translation-concealed rooted logical name that contains Perl and the logical device for the @INC path on VMS only 💜. Other logical names that affect Perl on VMS include PERLSHR, PERL_ENV_TABLES, and SYS$TIMEZONE_DIFFERENTIAL, but are optional and discussed further in perlvms and in README.vms in the Perl source distribution.

---

✨ PERL_SIGNALS

Available in Perls 5.8.1 and later 🎀. If set to "unsafe", the pre-Perl-5.8.0 signal behaviour (which is immediate but unsafe) is restored. If set to "safe", then safe (but deferred) signals are used 🌸. See "Deferred Signals (Safe Signals)" in perlipc.

---

🌸 PERL_UNICODE

Equivalent to the -C command-line switch. Note that this is not a boolean variable 🙅. Setting this to "1" is not the right way to "enable Unicode" (whatever that would mean 😭). You can use "0" to "disable Unicode", though (or alternatively unset PERL_UNICODE in your shell before starting Perl). See the description of the -C switch for more information.

---

💅 PERL_USE_UNSAFE_INC

If perl has been configured to not have the current directory in @INC by default, this variable can be set to "1" to reinstate it 👑. It's primarily intended for use while building and testing modules that have not been updated to deal with . not being in @INC and should not be set in the environment for day-to-day use. fr fr, leave it alone.

---

🧚 SYS$LOGIN (VMS port only)

Used if chdir has no argument and "HOME" and "LOGDIR" are not set. that's literally it 🫶

---

🎲 PERL_INTERNAL_RAND_SEED

Set to a non-negative integer to seed the random number generator used internally by perl for a variety of purposes 🌟.

Ignored if perl is run setuid or setgid. Used only for some limited startup randomization (hash keys) if -T or -t perl is started with tainting enabled.

Perl may be built to ignore this variable. 🙈

---

🔥 PERL_RAND_SEED

When set to an integer value this value will be used to seed the perl internal random number generator used for rand() when it is used without an explicit srand() call or for when an explicit no-argument srand() call is made 💜.

Normally calling rand() prior to calling srand() or calling srand() explicitly with no arguments should result in the random number generator using "best efforts" to seed the generator state with a relatively high quality random seed ✨. When this environment variable is set then the seeds used will be deterministically computed from the value provided in the env var in such a way that the application process and any forks or threads should continue to have their own unique seed, but that the program may be run twice with identical results as far as rand() goes (assuming all else is equal) 🎀.

PERL_RAND_SEED is intended for performance measurements and debugging and is explicitly NOT intended for stable testing 😤. The only guarantee is that a specific perl executable will produce the same results twice in a row — there is no guarantee that the results will be the same between perl releases or on different architectures 💀.

Ignored if perl is run setuid or setgid.

---

Perl also has environment variables that control how Perl handles data specific to particular natural languages; see perllocale 🌍.

Perl and its various modules and components, including its test frameworks, may sometimes make use of certain other environment variables. Some of these are specific to a particular platform 🛸. Please consult the appropriate module documentation and any documentation for your platform (like perlsolaris, perllinux, perlmacosx, perlwin32, etc) for variables peculiar to those specific situations.

Perl makes all environment variables available to the program being executed, and passes these along to any child processes it starts 👑. However, programs running setuid would do well to execute the following lines before doing anything else, just to keep people honest 🫶:

$ENV{PATH}  = "/bin:/usr/bin";    # or whatever you need
$ENV{SHELL} = "/bin/sh" if exists $ENV{SHELL};
delete @ENV{qw(IFS CDPATH ENV BASH_ENV)};

🎀 ORDER OF APPLICATION 🎀

okay so here's the tea ☕ — some options, in particular -I, -M, PERL5LIB and PERL5OPT, can interact with each other, and the order in which they are applied is important. like, genuinely. bestie, this is not a drill. 💅

heads up: this section does not document what actually happens inside the perl interpreter — it documents what effectively happens. vibe-level truth, not internal-lore-level truth. ✨

---

-I

the way that multiple -I options work: they get unshift-ed onto @INC from right to left. (backwards, yes, iconic behaviour 😌) so for example:

perl -I 1 -I 2 -I 3

will first prepend 3 onto the front of @INC, then prepend 2, and then prepend 1. the result is that @INC begins with:

qw(1 2 3)

-M

Multiple -M options are processed from left to right — so not like -I, they chose violence differently 💀. so this:

perl -Mlib=1 -Mlib=2 -Mlib=3

will first use the lib pragma to prepend 1 to @INC, then it will prepend 2, then it will prepend 3, resulting in an @INC that begins with:

qw(3 2 1)

the PERL5LIB environment variable

this one contains a list of directories, separated by colons. the entire list is prepended to @INC in one go — no drip-feed, all at once, main character entrance 🫶. so this:

PERL5LIB=1:2:3 perl

will result in an @INC that begins with:

qw(1 2 3)

combinations of -I, -M and PERL5LIB

when they all pull up to the same function, the order is: PERL5LIB is applied first, then all the -I arguments, then all the -M arguments. so this:

PERL5LIB=e1:e2 perl -I i1 -Mlib=m1 -I i2 -Mlib=m2

will result in an @INC that begins with:

qw(m2 m1 i1 i2 e1 e2)

the PERL5OPT environment variable

this contains a space separated list of switches. in this section we only consider the effects of -M and -I. (the others have their own lore, iykyk.)

After normal processing of -I switches from the command line, all the -I switches in PERL5OPT are extracted. They are processed from left to right instead of from right to left. Also note that while whitespace is allowed between a -I and its directory on the command line, it is not allowed in PERL5OPT. (rude, honestly 😭)

After normal processing of -M switches from the command line, all the -M switches in PERL5OPT are extracted. They are processed from left to right, i.e. the same as those on the command line.

An example may make this clearer:

export PERL5OPT="-Mlib=optm1 -Iopti1 -Mlib=optm2 -Iopti2"
export PERL5LIB=e1:e2
perl -I i1 -Mlib=m1 -I i2 -Mlib=m2

will result in an @INC that begins with:

qw(
    optm2
    optm1

    m2
    m1

    opti2
    opti1

    i1
    i2

    e1
    e2
)

Other complications

There are some complications that are ignored in the examples above:

arch and version subdirs

All of -I, PERL5LIB and use lib will also prepend arch and version subdirs if they are present

sitecustomize.pl

(the page just names this one and dips — no further tea given 🫖)

---

and that's a wrap bestie 💖 this was PERLRUN(1perl), from perl v5.44.0, dated 2026-10-17. now go forth and order your options correctly, you absolute icon 👑✨