PERLRUN(1)

NAME
perlrun - how to execute the Perl interpreter

SYNOPSIS
perl    [ -CsTtuUWX ]
    [ -hv ] [ -V[:configvar] ]
    [ -cw ] [ -d[:debugger] ] [ -D[number/list] ]
    [ -pna ] [ -Fpattern ] [ -l[octal] ] [ -0[octal] ]
    [ -Idir ] [ -m[-]module ] [ -M[-]'module...' ]
    [ -P ]
    [ -S ]
    [ -x[dir] ]
    [ -i[extension] ]
    [ -e 'command' ] [ -- ] [ programfile ] [ argument ]...

DESCRIPTION
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 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 behavior 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 -wS $0 ${1+"$@"}'
        if $running_under_some_shell;

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, perl5.005_57, you should place that directly in the #! line's path.

If the #! line does not contain the word "perl", 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 "ALTERNATIVE_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.

Macintosh
    A Macintosh perl program will have the appropriate Creator and Type, so that double-clicking them will invoke the perl application.

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\""

    # Macintosh
    print "Hello world\n"
     (then Run "Myscript" or Shift-Command-R)

    # 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.

Under the Macintosh, it depends which environment you are using. The MacPerl shell, or MPW, is much like Unix shells in its support for several quoting variants, except that it makes free use of the Macintosh's non-ASCII characters as control characters.

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/perl5.00554

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

    use 5.005_54;

  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

Switches include:

-0[digits]
    specifies the input record separator ($/) as an octal 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. The value 0777 will cause Perl to slurp files whole because there is no legal character with that value.

-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.

-C  enables Perl to use the native wide character APIs on the target system. The magic variable "${^WIDE_SYSTEM_CALLS}" reflects the state of this switch. See "${^WIDE_SYSTEM_CALLS}" in perlvar.

    This feature is currently only implemented on the Win32 platform.

-c  causes Perl to check the syntax of the program and then exit without executing it. Actually, it will execute "BEGIN", "CHECK", and "use" blocks, because these are considered as occurring outside the execution of your program. "INIT" and "END" blocks, however, will be skipped.

-d  runs the program under the Perl debugger. See perldebug.

-d:foo[=bar,baz]
    runs the program under the control of a debugging, profiling, or tracing module installed as Devel::foo. E.g., -d:DProf executes the program using the Devel::DProf profiler. As with the -M flag, options may be passed to the Devel::foo package where they will be received and interpreted by the Devel::foo::import routine. The comma-separated list of options must follow a "=" character. See perldebug.

-Dletters
-Dnumber
    sets debugging flags. To watch how it executes your program, use -Dtls. (This works only if debugging is compiled into your Perl.) 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 list of letters (e.g., -D14 is equivalent to -Dtls):

            1  p  Tokenizing and parsing
            2  s  Stack snapshots
            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, preprocessor command for -P, source file input state
          128  m  Memory allocation
          256  f  Format processing
          512  r  Regular expression parsing and execution
         1024  x  Syntax tree dump
         2048  u  Tainting checks
         4096  L  Memory leaks (needs -DLEAKTEST when compiling Perl)
         8192  H  Hash dump -- usurps values()
        16384  X  Scratchpad allocation
        32768  D  Cleaning up
        65536  S  Thread synchronization
       131072  T  Tokenising
       262144  R  Include reference counts of dumped variables (eg when using -Ds)
       524288  J  Do not s,t,P-debug (Jump over) opcodes within package DB

    All these flags require -DDEBUGGING when you compile the Perl executable (but see Devel::Peek, re which may change this). See the INSTALL file in the Perl source distribution for how to do this. This flag is automatically set if you include -g option when "Configure" asks you about optimizer/debugger flags.

    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 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.

-Fpattern
    specifies the pattern to split on if -a is also in effect. The pattern may be surrounded by "//", "", or '', otherwise it will be put in single quotes.

-h  prints a summary of the options.

-i[extension]
    specifies that files processed by the "<>" construct are to be edited in-place. It does this by renaming the input file, opening the output file by the original name, and selecting that output file as the default for print() statements. The extension, if supplied, is used to modify the name of the old file to make a backup copy, following these rules:

    If no extension is supplied, no backup is made and the current file is overwritten.

    If the extension doesn't contain a "*", then it is 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 terms, you could think of this as:

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

    This allows you to add a prefix to the backup file, instead of (or in addition to) a suffix:

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

    Or even to place backup copies of the original files into another directory (provided the directory already exists):

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

    These sets of one-liners are 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'

    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. Note that 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 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 want to append to each file, or reset line numbering (see 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).

    For a discussion of issues surrounding file permissions and -i, see "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.

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

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

    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), and also tells the C preprocessor where to search for include files. The C preprocessor is invoked with -P; by default it searches /usr/include and /usr/lib/perl.

-l[octnum]
    enables automatic line-ending processing. It has two separate effects. First, it automatically chomps $/ (the input record separator) when used with -n or -p. Second, it assigns "$\" (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, 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.

-m[-]module
-M[-]module
-M[-]'module ...'
-[mM][-]module=arg[,arg]...
    -mmodule executes "use" module "();" before executing your program.

    -Mmodule executes "use" module ";" before executing your program. 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'.

    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.

-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.

    Here is an efficient way to delete all files older than 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. 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.

-P  NOTE: Use of -P is strongly discouraged because of its inherent problems, including poor portability.

    This option causes your program to be run through the C preprocessor before compilation by Perl. Because both comments and cpp directives begin with the # character, you should avoid starting comments with any words recognized by the C preprocessor such as "if", "else", or "define".

    If you're considering using "-P", you might also want to look at the Filter::cpp module from CPAN.

    The problems of -P include, but are not limited to:

    •   The "#!" line is stripped, so any switches there don't apply.

    •   A "-P" on a "#!" line doesn't work.

    •   All lines that begin with (whitespace and) a "#" but do not look like cpp commands, are stripped, including anything inside Perl strings, regular expressions, and here-docs .

    •   In some platforms the C preprocessor knows too much: it knows about the C++ -style until-end-of-line comments starting with "//". This will cause problems with common Perl constructs like

            s/foo//;

        because after -P this will became illegal code

            s/foo

        The workaround is to use some other quoting separator than "/", like for example "!":

            s!foo!!;

    •   It requires not only a working C preprocessor but also a working sed. If not on UNIX, you are probably out of luck on this.

    •   Script line numbers are not preserved.

    •   The "-x" does not work with "-P".

-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 --). This means you can have switches with two leading dashes (--help). Any switch found there is removed from @ARGV and sets the corresponding variable in the Perl program. 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 --help creates the variable ${-help}, which is not compliant with "strict refs".

-S  makes Perl use the PATH environment variable to search for the program (unless the name of the program contains directory separators).

    On some platforms, this also makes Perl append suffixes to the filename while searching for it. For example, on Win32 platforms, the ".bat" and ".cmd" suffixes are appended if a lookup for the original name fails, and if the name does not already end in one of those suffixes. If your Perl was compiled with DEBUGGING turned on, using the -Dp switch to Perl shows how the search progresses.

    Typically this is used to emulate #! startup on platforms that don't support #!. 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 $running_under_some_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 variable $running_under_some_shell is never true. If the program will be interpreted by csh, you will need to replace "${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 $running_under_some_shell;

    If the filename supplied contains directory separators (i.e., 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.

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

    NOTE: this is not a substitute for -T. This is meant only to be used 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.

-T  forces "taint" checks to be turned on 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.

-u  This obsolete 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 dump() operator instead. Note: availability of undump is platform specific and may not be available for a specific port of Perl.

    This switch has been superseded in favor of the new Perl code generator backends to the compiler. See B and B::Bytecode for details.

-U  allows Perl to do unsafe operations. Currently the only "unsafe" operations are the unlinking of directories while running as superuser, and running setuid programs with fatal taint checks turned into warnings. Note that the -w switch (or the $^W variable) must be used 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:name
    Prints to STDOUT the value of the named configuration variable. For example,

        $ perl -V:man.dir

    will provide strong clues about what your MANPATH variable should be set to in order to access the Perl documentation.

-w  prints warnings about dubious constructs, such as variable names that are mentioned only once and scalar variables that are used before being set, redefined subroutines, references to undefined filehandles or filehandles opened read-only that you are attempting to write on, values used as a number that doesn'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 internal "^$W" variable. 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 new, fine-grained warning facility is also available if you want to manipulate entire classes of warnings; see warnings or perllexwarn.

-W  Enables all warnings regardless of "no warnings" or $^W. See perllexwarn.

-X  Disables all warnings regardless of "use warnings" or $^W. See perllexwarn.

-x directory
    tells Perl that the program is embedded in a larger chunk of unrelated ASCII 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. 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).

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 colon-separated list of directories in which to look for Perl library files before looking in the standard library and the current directory. Any architecture-specific directories under the specified locations are automatically included if they exist. If PERL5LIB is not defined, PERLLIB is used.

    When running taint checks (either because the program was running setuid or setgid, or the -T switch was used), neither variable is used. The program should instead say:

        use lib "/my/directory";

PERL5OPT
    Command-line options (switches). Switches in this variable are taken as if they were on every Perl command line. Only the -[DIMUdmtw] switches are allowed. When running taint checks (because the program was running setuid or setgid, or the -T switch was used), this variable is ignored. If PERL5OPT begins with -T, tainting will be enabled, and any subsequent options ignored.

PERLIO
    A space (or colon) separated list of PerlIO layers. If perl is built to use PerlIO system for IO (the default) these layers effect perl's IO.

    It is conventional to start layer names with a colon e.g. ":perlio" to emphasise their similarity to variable "attributes". But the code that parses layer specification strings (which is also used to decode the PERLIO environment variable) treats the colon as a separator.

    The list becomes the default for all perl's IO. Consequently only built-in layers can appear in this list, as external layers (such as :encoding()) need IO in order to load them!. See "open pragma" for how to add external encodings as defaults.

    The layers that it makes sense to include in the PERLIO environment variable are summarised below. For more details see PerlIO.

    :bytes
        Turns off the ":utf8" flag for the layer below. Unlikely to be useful in global PERLIO environment variable.

    :crlf
        A layer that implements DOS/Windows like CRLF line endings. On read converts pairs of CR,LF to a single "\n" newline character. On write converts each "\n" to a CR,LF pair. Based on the ":perlio" layer.

    :mmap
        A layer which implements "reading" of files by using "mmap()" to make (whole) file appear in the process's address space, and then using that as PerlIO's "buffer". This may be faster in certain circumstances for large files, and may result in less physical memory use when multiple processes are reading the same file.

        Files which are not "mmap()"-able revert to behaving like the ":perlio" layer. Writes also behave like ":perlio" layer as "mmap()" for write needs extra house-keeping (to extend the file) which negates any advantage.

        The ":mmap" layer will not exist if platform does not support "mmap()".

    :perlio
        A from scratch implementation of buffering for PerlIO. Provides fast access to the buffer for "sv_gets" which implements perl's readline/<> and in general attempts to minimize data copying.

        ":perlio" will insert a ":unix" layer below itself to do low level IO.

    :raw
        Applying the <:raw> layer is equivalent to calling "binmode($fh)". It makes the stream pass each byte as-is without any translation. In particular CRLF translation, and/or :utf8 inuited from locale are disabled.

        Arranges for all accesses go straight to the lowest buffered layer provided by the configration. That is it strips off any layers above that layer.

        In Perl 5.6 and some books the ":raw" layer (previously sometimes also referred to as a "discipline") is documented as the inverse of the ":crlf" layer. That is no longer the case - other layers which would alter binary nature of the stream are also disabled. If you want UNIX line endings on a platform that normally does CRLF translation, but still want UTF-8 or encoding defaults the appropriate thing to do is to add ":perlio" to PERLIO environment variable.

    :stdio
        This layer provides PerlIO interface by wrapping system's ANSI C "stdio" library calls. The layer provides both buffering and IO. Note that ":stdio" layer does not do CRLF translation even if that is platforms normal behaviour. You will need a ":crlf" layer above it to do that.

    :unix
        Lowest level layer which provides basic PerlIO operations in terms of UNIX/POSIX numeric file descriptor calls "open(), read(), write(), lseek(), close()"

    :utf8
        Turns on a flag on the layer below to tell perl that data sent to the stream should be converted to perl internal "utf8" form and that data from the stream should be considered as so encoded. On ASCII based platforms the encoding is UTF-8 and on EBCDIC platforms UTF-EBCDIC. May be useful in PERLIO environment variable to make UTF-8 the default. (To turn off that behaviour use ":bytes" layer.)

    :win32
        On Win32 platforms this experimental layer uses native "handle" IO rather than unix-like numeric file descriptor layer. Known to be buggy in this release.

    On all platforms the default set of layers should give acceptable results.

    For UNIX platforms that will equivalent of "unix perlio" or "stdio". Configure is setup to prefer "stdio" implementation if system's library provides for fast access to the buffer, otherwise it uses the "unix perlio" implementation.

    On Win32 the default in this release is "unix crlf". Win32's "stdio" has a number of bugs/mis-features for perl IO which are somewhat C compiler vendor/version dependent. Using our own "crlf" layer as the buffer avoids those issues and makes things more uniform. The "crlf" layer provides CRLF to/from "\n" conversion as well as buffering.

    This release uses "unix" as the bottom layer on Win32 and so still uses C compiler's numeric file descriptor routines. There is an experimental native "win32" layer which is expected to be enhanced and should eventually replace the "unix" layer.

PERLIO_DEBUG
    If set to the name of a file or device then certain operations of PerlIO sub-system will be logged to that file (opened as append). Typical uses are UNIX:

       PERLIO_DEBUG=/dev/tty perl script ...

    and Win32 approximate equivalent:

       set PERLIO_DEBUG=CON
       perl script ...

PERLLIB
    A colon-separated list of directories in which to look for Perl library files before looking in the standard library and the current directory. If PERL5LIB is defined, PERLLIB is not used.

PERL5DB
    The command used to load the debugger code. The default is:

            BEGIN { require 'perl5db.pl' }

PERL5SHELL (specific to the Win32 port)
    May be set to an alternative shell that perl must use internally for executing "backtick" commands or system(). Default is "cmd.exe /x/c" on WindowsNT and "command.com /c" on Windows95. The value is considered to be space-separated. Precede any character that needs to be protected (like a space or backslash) with a backslash.

    Note that Perl doesn't use COMSPEC for this purpose because COMSPEC has a high degree of variability among users, leading to portability concerns. Besides, perl can use a shell that may not be fit for interactive use, and setting COMSPEC to such a shell may interfere with the proper functioning of other programs (which usually look in COMSPEC to find a shell fit for interactive use).

PERL_DEBUG_MSTATS
    Relevant only if perl is compiled with the malloc included with the perl distribution (that is, if "perl -V:d_mymalloc" is 'define'). If set, this causes memory statistics to be dumped after execution. If set to an integer greater than one, also causes memory statistics to be dumped after compilation.

PERL_DESTRUCT_LEVEL
    Relevant only if your perl executable was built with -DDEBUGGING, this controls the behavior of global destruction of objects and other references. See "PERL_DESTRUCT_LEVEL" in perlhack for more information.

PERL_ENCODING
    If using the "encoding" pragma without an explicit encoding name, the PERL_ENCODING environment variable is consulted for an encoding name.

PERL_ROOT (specific to the VMS port)
    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.

SYS$LOGIN (specific to the VMS port)
    Used if chdir has no argument and HOME and LOGDIR are not set.

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

Apart from these, Perl uses no other environment variables, except to make them available to the program being executed, and to child processes. 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)};