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.\" ========================================================================
.\"
.IX Title "PERLRUN 1"
.TH PERLRUN 1 "2013-03-04" "perl v5.16.3" "Perl Programmers Reference Guide"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
perlrun \- how to execute the Perl interpreter
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
\&\fBperl\fR	[\ \fB\-sTtuUWX\fR\ ]
	[\ \fB\-hv\fR\ ]\ [\ \fB\-V\fR[:\fIconfigvar\fR]\ ]
	[\ \fB\-cw\fR\ ]\ [\ \fB\-d\fR[\fBt\fR][:\fIdebugger\fR]\ ]\ [\ \fB\-D\fR[\fInumber/list\fR]\ ]
	[\ \fB\-pna\fR\ ]\ [\ \fB\-F\fR\fIpattern\fR\ ]\ [\ \fB\-l\fR[\fIoctal\fR]\ ]\ [\ \fB\-0\fR[\fIoctal/hexadecimal\fR]\ ]
	[\ \fB\-I\fR\fIdir\fR\ ]\ [\ \fB\-m\fR[\fB\-\fR]\fImodule\fR\ ]\ [\ \fB\-M\fR[\fB\-\fR]\fI'module...'\fR\ ]\ [\ \fB\-f\fR\ ]
	[\ \fB\-C\ [\f(BInumber/list\fB]\ \fR]
	[\ \fB\-S\fR\ ]
	[\ \fB\-x\fR[\fIdir\fR]\ ]
	[\ \fB\-i\fR[\fIextension\fR]\ ]
	[\ [\fB\-e\fR|\fB\-E\fR]\ \fI'command'\fR\ ]\ [\ \fB\-\-\fR\ ]\ [\ \fIprogramfile\fR\ ]\ [\ \fIargument\fR\ ]...
.SH "DESCRIPTION"
.IX Header "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:
.IP "1." 4
Specified line by line via \fB\-e\fR or \fB\-E\fR switches on the command line.
.IP "2." 4
Contained in the file specified by the first filename on the command line.
(Note that systems supporting the \f(CW\*(C`#!\*(C'\fR notation invoke interpreters this
way. See \*(L"Location of Perl\*(R".)
.IP "3." 4
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 \*(L"\-\*(R" for the program name.
.PP
With methods 2 and 3, Perl starts parsing the input file from the
beginning, unless you've specified a \fB\-x\fR switch, in which case it
scans for the first line starting with \f(CW\*(C`#!\*(C'\fR and containing the word
\&\*(L"perl\*(R", 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 \f(CW\*(C`_\|_END_\|_\*(C'\fR token.)
.PP
The \f(CW\*(C`#!\*(C'\fR 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 \f(CW\*(C`#!\*(C'\fR line, or worse, doesn't even recognize the \f(CW\*(C`#!\*(C'\fR line, you
still can get consistent switch behaviour regardless of how Perl was
invoked, even if \fB\-x\fR was used to find the beginning of the program.
.PP
Because historically some operating systems silently chopped off
kernel interpretation of the \f(CW\*(C`#!\*(C'\fR line after 32 characters, some
switches may be passed in on the command line, and some may not;
you could even get a \*(L"\-\*(R" 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 \*(L"\-\*(R"
instead of a complete switch could cause Perl to try to execute
standard input instead of your program.  And a partial \fB\-I\fR switch
could also cause odd results.
.PP
Some switches do care if they are processed twice, for instance
combinations of \fB\-l\fR and \fB\-0\fR.  Either put all the switches after
the 32\-character boundary (if applicable), or replace the use of
\&\fB\-0\fR\fIdigits\fR by \f(CW\*(C`BEGIN{ $/ = "\e0digits"; }\*(C'\fR.
.PP
Parsing of the \f(CW\*(C`#!\*(C'\fR switches starts wherever \*(L"perl\*(R" is mentioned in the line.
The sequences \*(L"\-*\*(R" and \*(L"\- \*(R" are specifically ignored so that you could,
if you were so inclined, say
.PP
.Vb 4
\&    #!/bin/sh
\&    #! \-*\-perl\-*\-
\&    eval \*(Aqexec perl \-x \-wS $0 ${1+"$@"}\*(Aq
\&        if 0;
.Ve
.PP
to let Perl see the \fB\-p\fR switch.
.PP
A similar trick involves the \fIenv\fR program, if you have it.
.PP
.Vb 1
\&    #!/usr/bin/env perl
.Ve
.PP
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 \f(CW\*(C`#!\*(C'\fR line's path.
.PP
If the \f(CW\*(C`#!\*(C'\fR line does not contain the word \*(L"perl\*(R" nor the word \*(L"indir\*(R"
the program named after the \f(CW\*(C`#!\*(C'\fR is executed instead of the Perl
interpreter.  This is slightly bizarre, but it helps people on machines
that don't do \f(CW\*(C`#!\*(C'\fR, because they can tell a program that their \s-1SHELL\s0 is
\&\fI/usr/bin/perl\fR, and Perl will then dispatch the program to the correct
interpreter for them.
.PP
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.)
.PP
If the program is syntactically correct, it is executed.  If the program
runs off the end without hitting an \fIexit()\fR or \fIdie()\fR operator, an implicit
\&\f(CWexit(0)\fR is provided to indicate successful completion.
.SS "#! and quoting on non-Unix systems"
.IX Xref "hashbang #!"
.IX Subsection "#! and quoting on non-Unix systems"
Unix's \f(CW\*(C`#!\*(C'\fR technique can be simulated on other systems:
.IP "\s-1OS/2\s0" 4
.IX Item "OS/2"
Put
.Sp
.Vb 1
\&    extproc perl \-S \-your_switches
.Ve
.Sp
as the first line in \f(CW\*(C`*.cmd\*(C'\fR file (\fB\-S\fR due to a bug in cmd.exe's
`extproc' handling).
.IP "MS-DOS" 4
.IX Item "MS-DOS"
Create a batch file to run your program, and codify it in
\&\f(CW\*(C`ALTERNATE_SHEBANG\*(C'\fR (see the \fIdosish.h\fR file in the source
distribution for more information).
.IP "Win95/NT" 4
.IX Item "Win95/NT"
The Win95/NT installation, when using the ActiveState installer for Perl,
will modify the Registry to associate the \fI.pl\fR 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.
.IP "\s-1VMS\s0" 4
.IX Item "VMS"
Put
.Sp
.Vb 2
\& $ perl \-mysw \*(Aqf$env("procedure")\*(Aq \*(Aqp1\*(Aq \*(Aqp2\*(Aq \*(Aqp3\*(Aq \*(Aqp4\*(Aq \*(Aqp5\*(Aq \*(Aqp6\*(Aq \*(Aqp7\*(Aq \*(Aqp8\*(Aq !
\& $ exit++ + ++$status != 0 and $exit = $status = undef;
.Ve
.Sp
at the top of your program, where \fB\-mysw\fR are any command line switches you
want to pass to Perl.  You can now invoke the program directly, by saying
\&\f(CW\*(C`perl program\*(C'\fR, or as a \s-1DCL\s0 procedure, by saying \f(CW@program\fR (or implicitly
via \fI\s-1DCL$PATH\s0\fR by just using the name of the program).
.Sp
This incantation is a bit much to remember, but Perl will display it for
you if you say \f(CW\*(C`perl "\-V:startperl"\*(C'\fR.
.PP
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 (\f(CW\*(C`*\*(C'\fR, \f(CW\*(C`\e\*(C'\fR and \f(CW\*(C`"\*(C'\fR are
common) and how to protect whitespace and these characters to run
one-liners (see \-e below).
.PP
On some systems, you may have to change single-quotes to double ones,
which you must \fInot\fR do on Unix or Plan 9 systems.  You might also
have to change a single % to a %%.
.PP
For example:
.PP
.Vb 2
\&    # Unix
\&    perl \-e \*(Aqprint "Hello world\en"\*(Aq
\&
\&    # MS\-DOS, etc.
\&    perl \-e "print \e"Hello world\en\e""
\&
\&    # VMS
\&    perl \-e "print ""Hello world\en"""
.Ve
.PP
The problem is that none of this is reliable: it depends on the
command and it is entirely possible neither works.  If \fI4DOS\fR were
the command shell, this would probably work better:
.PP
.Vb 1
\&    perl \-e "print <Ctrl\-x>"Hello world\en<Ctrl\-x>""
.Ve
.PP
\&\fB\s-1CMD\s0.EXE\fR in Windows \s-1NT\s0 slipped a lot of standard Unix functionality in
when nobody was looking, but just try to find documentation for its
quoting rules.
.PP
There is no general solution to all of this.  It's just a mess.
.SS "Location of Perl"
.IX Xref "perl, location of interpreter"
.IX Subsection "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 \fI/usr/bin/perl\fR
and \fI/usr/local/bin/perl\fR 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 \s-1PATH\s0, or in some other
obvious and convenient place.
.PP
In this documentation, \f(CW\*(C`#!/usr/bin/perl\*(C'\fR 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.
.PP
.Vb 1
\&    #!/usr/local/bin/perl5.00554
.Ve
.PP
or if you just want to be running at least version, place a statement
like this at the top of your program:
.PP
.Vb 1
\&    use 5.005_54;
.Ve
.SS "Command Switches"
.IX Xref "perl, command switches command switches"
.IX Subsection "Command Switches"
As with all standard commands, a single-character switch may be
clustered with the following switch, if any.
.PP
.Vb 1
\&    #!/usr/bin/perl \-spi.orig   # same as \-s \-p \-i.orig
.Ve
.PP
Switches include:
.IP "\fB\-0\fR[\fIoctal/hexadecimal\fR]" 5
.IX Xref "-0 $"
.IX Item "-0[octal/hexadecimal]"
specifies the input record separator (\f(CW$/\fR) 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 \fIfind\fR which can print filenames
terminated by the null character, you can say this:
.Sp
.Vb 1
\&    find . \-name \*(Aq*.orig\*(Aq \-print0 | perl \-n0e unlink
.Ve
.Sp
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.
.Sp
You can also specify the separator character using hexadecimal notation:
\&\fB\-0x\f(BI\s-1HHH\s0...\fB\fR, where the \f(CW\*(C`\f(CIH\f(CW\*(C'\fR 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 \fIreally\fR want a record separator of 0777,
specify it as \fB\-0x1FF\fR.  (This means that you cannot use the \fB\-x\fR option
with a directory name that consists of hexadecimal digits, or else Perl
will think you have specified a hex number to \fB\-0\fR.)
.IP "\fB\-a\fR" 5
.IX Xref "-a autosplit"
.IX Item "-a"
turns on autosplit mode when used with a \fB\-n\fR or \fB\-p\fR.  An implicit
split command to the \f(CW@F\fR array is done as the first thing inside the
implicit while loop produced by the \fB\-n\fR or \fB\-p\fR.
.Sp
.Vb 1
\&    perl \-ane \*(Aqprint pop(@F), "\en";\*(Aq
.Ve
.Sp
is equivalent to
.Sp
.Vb 4
\&    while (<>) {
\&        @F = split(\*(Aq \*(Aq);
\&        print pop(@F), "\en";
\&    }
.Ve
.Sp
An alternate delimiter may be specified using \fB\-F\fR.
.IP "\fB\-C [\f(BInumber/list\fB]\fR" 5
.IX Xref "-C"
.IX Item "-C [number/list]"
The \fB\-C\fR flag controls some of the Perl Unicode features.
.Sp
As of 5.8.1, the \fB\-C\fR can be followed either by a number or a list
of option letters.  The letters, their numeric values, and effects
are as follows; listing the letters is equal to summing the numbers.
.Sp
.Vb 10
\&    I     1   STDIN is assumed to be in UTF\-8
\&    O     2   STDOUT will be in UTF\-8
\&    E     4   STDERR will be in UTF\-8
\&    S     7   I + O + E
\&    i     8   UTF\-8 is the default PerlIO layer for input streams
\&    o    16   UTF\-8 is the default PerlIO layer for output streams
\&    D    24   i + o
\&    A    32   the @ARGV elements are expected to be strings encoded
\&              in UTF\-8
\&    L    64   normally the "IOEioA" are unconditional, the L makes
\&              them conditional on the locale environment variables
\&              (the LC_ALL, LC_TYPE, and LANG, in the order of
\&              decreasing precedence) \-\- if the variables indicate
\&              UTF\-8, then the selected "IOEioA" are in effect
\&    a   256   Set ${^UTF8CACHE} to \-1, to run the UTF\-8 caching
\&              code in debugging mode.
.Ve
.Sp
For example, \fB\-COE\fR and \fB\-C6\fR will both turn on UTF\-8\-ness on both
\&\s-1STDOUT\s0 and \s-1STDERR\s0.  Repeating letters is just redundant, not cumulative
nor toggling.
.Sp
The \f(CW\*(C`io\*(C'\fR options mean that any subsequent \fIopen()\fR (or similar I/O
operations) in the current file scope will have the \f(CW\*(C`:utf8\*(C'\fR PerlIO layer
implicitly applied to them, in other words, \s-1UTF\-8\s0 is expected from any
input stream, and \s-1UTF\-8\s0 is produced to any output stream.  This is just
the default, with explicit layers in \fIopen()\fR and with \fIbinmode()\fR one can
manipulate streams as usual.
.Sp
\&\fB\-C\fR on its own (not followed by any number or option list), or the
empty string \f(CW""\fR for the \f(CW\*(C`PERL_UNICODE\*(C'\fR environment variable, has the
same effect as \fB\-CSDL\fR.  In other words, the standard I/O handles and
the default \f(CW\*(C`open()\*(C'\fR layer are UTF\-8\-fied \fIbut\fR only if the locale
environment variables indicate a \s-1UTF\-8\s0 locale.  This behaviour follows
the \fIimplicit\fR (and problematic) \s-1UTF\-8\s0 behaviour of Perl 5.8.0.
(See \*(L"\s-1UTF\-8\s0 no longer default under \s-1UTF\-8\s0 locales\*(R" in perl581delta.)
.Sp
You can use \fB\-C0\fR (or \f(CW"0"\fR for \f(CW\*(C`PERL_UNICODE\*(C'\fR) to explicitly
disable all the above Unicode features.
.Sp
The read-only magic variable \f(CW\*(C`${^UNICODE}\*(C'\fR reflects the numeric value
of this setting.  This variable is set during Perl startup and is
thereafter read-only.  If you want runtime effects, use the three-arg
\&\fIopen()\fR (see \*(L"open\*(R" in perlfunc), the two-arg \fIbinmode()\fR (see \*(L"binmode\*(R" in perlfunc),
and the \f(CW\*(C`open\*(C'\fR pragma (see open).
.Sp
(In Perls earlier than 5.8.1 the \fB\-C\fR switch was a Win32\-only switch
that enabled the use of Unicode-aware \*(L"wide system call\*(R" Win32 APIs.
This feature was practically unused, however, and the command line
switch was therefore \*(L"recycled\*(R".)
.Sp
\&\fBNote:\fR Since perl 5.10.1, if the \fB\-C\fR option is used on the \f(CW\*(C`#!\*(C'\fR 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 \fIbinmode()\fR to set the encoding of an I/O stream.
.IP "\fB\-c\fR" 5
.IX Xref "-c"
.IX Item "-c"
causes Perl to check the syntax of the program and then exit without
executing it.  Actually, it \fIwill\fR execute and \f(CW\*(C`BEGIN\*(C'\fR, \f(CW\*(C`UNITCHECK\*(C'\fR,
or \f(CW\*(C`CHECK\*(C'\fR blocks and any \f(CW\*(C`use\*(C'\fR statements: these are considered as
occurring outside the execution of your program.  \f(CW\*(C`INIT\*(C'\fR and \f(CW\*(C`END\*(C'\fR
blocks, however, will be skipped.
.IP "\fB\-d\fR" 5
.IX Xref "-d -dt"
.IX Item "-d"
.PD 0
.IP "\fB\-dt\fR" 5
.IX Item "-dt"
.PD
runs the program under the Perl debugger.  See perldebug.
If \fBt\fR is specified, it indicates to the debugger that threads
will be used in the code being debugged.
.IP "\fB\-d:\fR\fIMOD[=bar,baz]\fR" 5
.IX Xref "-d -dt"
.IX Item "-d:MOD[=bar,baz]"
.PD 0
.IP "\fB\-dt:\fR\fIMOD[=bar,baz]\fR" 5
.IX Item "-dt:MOD[=bar,baz]"
.PD
runs the program under the control of a debugging, profiling, or tracing
module installed as \f(CW\*(C`Devel::\f(CIMOD\f(CW\*(C'\fR. E.g., \fB\-d:DProf\fR executes the
program using the \f(CW\*(C`Devel::DProf\*(C'\fR profiler.  As with the \fB\-M\fR flag, options
may be passed to the \f(CW\*(C`Devel::\f(CIMOD\f(CW\*(C'\fR package where they will be received
and interpreted by the \f(CW\*(C`Devel::\f(CIMOD\f(CW::import\*(C'\fR routine.  Again, like \fB\-M\fR,
use \-\fB\-d:\-\f(BI\s-1MOD\s0\fB\fR to call \f(CW\*(C`Devel::\f(CIMOD\f(CW::unimport\*(C'\fR instead of import.  The
comma-separated list of options must follow a \f(CW\*(C`=\*(C'\fR character.  If \fBt\fR is
specified, it indicates to the debugger that threads will be used in the
code being debugged.  See perldebug.
.IP "\fB\-D\fR\fIletters\fR" 5
.IX Xref "-D DEBUGGING -DDEBUGGING"
.IX Item "-Dletters"
.PD 0
.IP "\fB\-D\fR\fInumber\fR" 5
.IX Item "-Dnumber"
.PD
sets debugging flags.  To watch how it executes your program, use
\&\fB\-Dtls\fR.  (This works only if debugging is compiled into your
Perl.)  Another nice value is \fB\-Dx\fR, which lists your compiled
syntax tree.  And \fB\-Dr\fR displays compiled regular expressions;
the format of the output is explained in perldebguts.
.Sp
As an alternative, specify a number instead of list of letters (e.g.,
\&\fB\-D14\fR is equivalent to \fB\-Dtls\fR):
.Sp
.Vb 10
\&        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  Hash dump \-\- usurps values()
\&    16384  X  Scratchpad allocation
\&    32768  D  Cleaning up
\&   131072  T  Tokenizing
\&   262144  R  Include reference counts of dumped variables (eg when
\&              using \-Ds)
\&   524288  J  show s,t,P\-debug (don\*(Aqt Jump over) on opcodes within
\&              package DB
\&  1048576  v  Verbose: use in conjunction with 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
.Ve
.Sp
All these flags require \fB\-DDEBUGGING\fR when you compile the Perl
executable (but see \f(CW\*(C`:opd\*(C'\fR in Devel::Peek or \*(L"'debug' mode\*(R" in re
which may change this).
See the \fI\s-1INSTALL\s0\fR file in the Perl source distribution
for how to do this.  This flag is automatically set if you include \fB\-g\fR
option when \f(CW\*(C`Configure\*(C'\fR asks you about optimizer/debugger flags.
.Sp
If you're just trying to get a print out of each line of Perl code
as it executes, the way that \f(CW\*(C`sh \-x\*(C'\fR provides for shell scripts,
you can't use Perl's \fB\-D\fR switch.  Instead do this
.Sp
.Vb 2
\&  # 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)
.Ve
.Sp
See perldebug for details and variations.
.IP "\fB\-e\fR \fIcommandline\fR" 5
.IX Xref "-e"
.IX Item "-e commandline"
may be used to enter one line of program.  If \fB\-e\fR is given, Perl
will not look for a filename in the argument list.  Multiple \fB\-e\fR
commands may be given to build up a multi-line script.  Make sure
to use semicolons where you would in a normal program.
.IP "\fB\-E\fR \fIcommandline\fR" 5
.IX Xref "-E"
.IX Item "-E commandline"
behaves just like \fB\-e\fR, except that it implicitly enables all
optional features (in the main compilation unit). See feature.
.IP "\fB\-f\fR" 5
.IX Xref "-f sitecustomize sitecustomize.pl"
.IX Item "-f"
Disable executing \fI\f(CI$Config\fI{sitelib}/sitecustomize.pl\fR at startup.
.Sp
Perl can be built so that it by default will try to execute
\&\fI\f(CI$Config\fI{sitelib}/sitecustomize.pl\fR at startup (in a \s-1BEGIN\s0 block).
This is a hook that allows the sysadmin to customize how Perl behaves.
It can for instance be used to add entries to the \f(CW@INC\fR array to make Perl
find modules in non-standard locations.
.Sp
Perl actually inserts the following code:
.Sp
.Vb 4
\&    BEGIN {
\&        do { local $!; \-f "$Config{sitelib}/sitecustomize.pl"; }
\&            && do "$Config{sitelib}/sitecustomize.pl";
\&    }
.Ve
.Sp
Since it is an actual \f(CW\*(C`do\*(C'\fR (not a \f(CW\*(C`require\*(C'\fR), \fIsitecustomize.pl\fR
doesn't need to return a true value. The code is run in package \f(CW\*(C`main\*(C'\fR,
in its own lexical scope. However, if the script dies, \f(CW$@\fR will not
be set.
.Sp
The value of \f(CW$Config{sitelib}\fR is also determined in C code and not
read from \f(CW\*(C`Config.pm\*(C'\fR, which is not loaded.
.Sp
The code is executed \fIvery\fR early. For example, any changes made to
\&\f(CW@INC\fR will show up in the output of `perl \-V`. Of course, \f(CW\*(C`END\*(C'\fR
blocks will be likewise executed very late.
.Sp
To determine at runtime if this capability has been compiled in your
perl, you can check the value of \f(CW$Config{usesitecustomize}\fR.
.IP "\fB\-F\fR\fIpattern\fR" 5
.IX Xref "-F"
.IX Item "-Fpattern"
specifies the pattern to split on if \fB\-a\fR is also in effect.  The
pattern may be surrounded by \f(CW\*(C`//\*(C'\fR, \f(CW""\fR, or \f(CW\*(Aq\*(Aq\fR, otherwise it will be
put in single quotes. You can't use literal whitespace in the pattern.
.IP "\fB\-h\fR" 5
.IX Xref "-h"
.IX Item "-h"
prints a summary of the options.
.IP "\fB\-i\fR[\fIextension\fR]" 5
.IX Xref "-i in-place"
.IX Item "-i[extension]"
specifies that files processed by the \f(CW\*(C`<>\*(C'\fR 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 \fIprint()\fR statements.  The extension, if supplied, is used to
modify the name of the old file to make a backup copy, following these
rules:
.Sp
If no extension is supplied, no backup is made and the current file is
overwritten.
.Sp
If the extension doesn't contain a \f(CW\*(C`*\*(C'\fR, then it is appended to the
end of the current filename as a suffix.  If the extension does
contain one or more \f(CW\*(C`*\*(C'\fR characters, then each \f(CW\*(C`*\*(C'\fR is replaced
with the current filename.  In Perl terms, you could think of this
as:
.Sp
.Vb 1
\&    ($backup = $extension) =~ s/\e*/$file_name/g;
.Ve
.Sp
This allows you to add a prefix to the backup file, instead of (or in
addition to) a suffix:
.Sp
.Vb 2
\& $ perl \-pi\*(Aqorig_*\*(Aq \-e \*(Aqs/bar/baz/\*(Aq fileA  # backup to
\&                                           # \*(Aqorig_fileA\*(Aq
.Ve
.Sp
Or even to place backup copies of the original files into another
directory (provided the directory already exists):
.Sp
.Vb 2
\& $ perl \-pi\*(Aqold/*.orig\*(Aq \-e \*(Aqs/bar/baz/\*(Aq fileA  # backup to
\&                                               # \*(Aqold/fileA.orig\*(Aq
.Ve
.Sp
These sets of one-liners are equivalent:
.Sp
.Vb 2
\& $ perl \-pi \-e \*(Aqs/bar/baz/\*(Aq fileA          # overwrite current file
\& $ perl \-pi\*(Aq*\*(Aq \-e \*(Aqs/bar/baz/\*(Aq fileA       # overwrite current file
\&
\& $ perl \-pi\*(Aq.orig\*(Aq \-e \*(Aqs/bar/baz/\*(Aq fileA   # backup to \*(AqfileA.orig\*(Aq
\& $ perl \-pi\*(Aq*.orig\*(Aq \-e \*(Aqs/bar/baz/\*(Aq fileA  # backup to \*(AqfileA.orig\*(Aq
.Ve
.Sp
From the shell, saying
.Sp
.Vb 1
\&    $ perl \-p \-i.orig \-e "s/foo/bar/; ... "
.Ve
.Sp
is the same as using the program:
.Sp
.Vb 2
\&    #!/usr/bin/perl \-pi.orig
\&    s/foo/bar/;
.Ve
.Sp
which is equivalent to
.Sp
.Vb 10
\&    #!/usr/bin/perl
\&    $extension = \*(Aq.orig\*(Aq;
\&    LINE: while (<>) {
\&        if ($ARGV ne $oldargv) {
\&            if ($extension !~ /\e*/) {
\&                $backup = $ARGV . $extension;
\&            }
\&            else {
\&                ($backup = $extension) =~ s/\e*/$ARGV/g;
\&            }
\&            rename($ARGV, $backup);
\&            open(ARGVOUT, ">$ARGV");
\&            select(ARGVOUT);
\&            $oldargv = $ARGV;
\&        }
\&        s/foo/bar/;
\&    }
\&    continue {
\&        print;  # this prints to original filename
\&    }
\&    select(STDOUT);
.Ve
.Sp
except that the \fB\-i\fR form doesn't need to compare \f(CW$ARGV\fR to \f(CW$oldargv\fR to
know when the filename has changed.  It does, however, use \s-1ARGVOUT\s0 for
the selected filehandle.  Note that \s-1STDOUT\s0 is restored as the default
output filehandle after the loop.
.Sp
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:
.Sp
.Vb 3
\&    $ perl \-p \-i\*(Aq/some/file/path/*\*(Aq \-e 1 file1 file2 file3...
\&or
\&    $ perl \-p \-i\*(Aq.orig\*(Aq \-e 1 file1 file2 file3...
.Ve
.Sp
You can use \f(CW\*(C`eof\*(C'\fR 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 \*(L"eof\*(R" in perlfunc).
.Sp
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).
.Sp
For a discussion of issues surrounding file permissions and \fB\-i\fR,
see \*(L"Why does Perl let me delete read-only files?  Why does \-i clobber protected files?  Isn't this a bug in Perl?\*(R" in perlfaq5.
.Sp
You cannot use \fB\-i\fR to create directories or to strip extensions from
files.
.Sp
Perl does not expand \f(CW\*(C`~\*(C'\fR in filenames, which is good, since some
folks use it for their backup files:
.Sp
.Vb 1
\&    $ perl \-pi~ \-e \*(Aqs/foo/bar/\*(Aq file1 file2 file3...
.Ve
.Sp
Note that because \fB\-i\fR 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.
.Sp
Finally, the \fB\-i\fR 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 \s-1STDIN\s0 to \s-1STDOUT\s0 as might be expected.
.IP "\fB\-I\fR\fIdirectory\fR" 5
.IX Xref "-I @INC"
.IX Item "-Idirectory"
Directories specified by \fB\-I\fR are prepended to the search path for
modules (\f(CW@INC\fR).
.IP "\fB\-l\fR[\fIoctnum\fR]" 5
.IX Xref "-l $ $\\"
.IX Item "-l[octnum]"
enables automatic line-ending processing.  It has two separate
effects.  First, it automatically chomps \f(CW$/\fR (the input record
separator) when used with \fB\-n\fR or \fB\-p\fR.  Second, it assigns \f(CW\*(C`$\e\*(C'\fR
(the output record separator) to have the value of \fIoctnum\fR so
that any print statements will have that separator added back on.
If \fIoctnum\fR is omitted, sets \f(CW\*(C`$\e\*(C'\fR to the current value of
\&\f(CW$/\fR.  For instance, to trim lines to 80 columns:
.Sp
.Vb 1
\&    perl \-lpe \*(Aqsubstr($_, 80) = ""\*(Aq
.Ve
.Sp
Note that the assignment \f(CW\*(C`$\e = $/\*(C'\fR is done when the switch is processed,
so the input record separator can be different than the output record
separator if the \fB\-l\fR switch is followed by a \fB\-0\fR switch:
.Sp
.Vb 1
\&    gnufind / \-print0 | perl \-ln0e \*(Aqprint "found $_" if \-p\*(Aq
.Ve
.Sp
This sets \f(CW\*(C`$\e\*(C'\fR to newline and then sets \f(CW$/\fR to the null character.
.IP "\fB\-m\fR[\fB\-\fR]\fImodule\fR" 5
.IX Xref "-m -M"
.IX Item "-m[-]module"
.PD 0
.IP "\fB\-M\fR[\fB\-\fR]\fImodule\fR" 5
.IX Item "-M[-]module"
.IP "\fB\-M\fR[\fB\-\fR]\fI'module ...'\fR" 5
.IX Item "-M[-]'module ...'"
.IP "\fB\-[mM]\fR[\fB\-\fR]\fImodule=arg[,arg]...\fR" 5
.IX Item "-[mM][-]module=arg[,arg]..."
.PD
\&\fB\-m\fR\fImodule\fR executes \f(CW\*(C`use\*(C'\fR \fImodule\fR \f(CW\*(C`();\*(C'\fR before executing your
program.
.Sp
\&\fB\-M\fR\fImodule\fR executes \f(CW\*(C`use\*(C'\fR \fImodule\fR \f(CW\*(C`;\*(C'\fR before executing your
program.  You can use quotes to add extra code after the module name,
e.g., \f(CW\*(Aq\-M\f(CIMODULE\f(CW qw(foo bar)\*(Aq\fR.
.Sp
If the first character after the \fB\-M\fR or \fB\-m\fR is a dash (\fB\-\fR)
then the 'use' is replaced with 'no'.
.Sp
A little builtin syntactic sugar means you can also say
\&\fB\-m\f(BI\s-1MODULE\s0\fB=foo,bar\fR or \fB\-M\f(BI\s-1MODULE\s0\fB=foo,bar\fR as a shortcut for
\&\fB'\-M\f(BI\s-1MODULE\s0\fB qw(foo bar)'\fR.  This avoids the need to use quotes when
importing symbols.  The actual code generated by \fB\-M\f(BI\s-1MODULE\s0\fB=foo,bar\fR is
\&\f(CW\*(C`use module split(/,/,q{foo,bar})\*(C'\fR.  Note that the \f(CW\*(C`=\*(C'\fR form
removes the distinction between \fB\-m\fR and \fB\-M\fR.
.Sp
A consequence of this is that \fB\-M\f(BI\s-1MODULE\s0\fB=number\fR never does a version check,
unless \f(CW\*(C`\f(CIMODULE\f(CW::import()\*(C'\fR itself is set up to do a version check, which
could happen for example if \fI\s-1MODULE\s0\fR inherits from Exporter.
.IP "\fB\-n\fR" 5
.IX Xref "-n"
.IX Item "-n"
causes Perl to assume the following loop around your program, which
makes it iterate over filename arguments somewhat like \fIsed \-n\fR or
\&\fIawk\fR:
.Sp
.Vb 4
\&  LINE:
\&    while (<>) {
\&        ...             # your program goes here
\&    }
.Ve
.Sp
Note that the lines are not printed by default.  See \*(L"\-p\*(R" 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.
.Sp
Also note that \f(CW\*(C`<>\*(C'\fR passes command line arguments to
\&\*(L"open\*(R" in perlfunc, which doesn't necessarily interpret them as file names.
See  perlop for possible security implications.
.Sp
Here is an efficient way to delete all files that haven't been modified for
at least a week:
.Sp
.Vb 1
\&    find . \-mtime +7 \-print | perl \-nle unlink
.Ve
.Sp
This is faster than using the \fB\-exec\fR switch of \fIfind\fR 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 \fB\-0\fR.
.Sp
\&\f(CW\*(C`BEGIN\*(C'\fR and \f(CW\*(C`END\*(C'\fR blocks may be used to capture control before or after
the implicit program loop, just as in \fIawk\fR.
.IP "\fB\-p\fR" 5
.IX Xref "-p"
.IX Item "-p"
causes Perl to assume the following loop around your program, which
makes it iterate over filename arguments somewhat like \fIsed\fR:
.Sp
.Vb 6
\&  LINE:
\&    while (<>) {
\&        ...             # your program goes here
\&    } continue {
\&        print or die "\-p destination: $!\en";
\&    }
.Ve
.Sp
If a file named by an argument cannot be opened for some reason, Perl
warns you about it, and moves on to the next file.  Note that the
lines are printed automatically.  An error occurring during printing is
treated as fatal.  To suppress printing use the \fB\-n\fR switch.  A \fB\-p\fR
overrides a \fB\-n\fR switch.
.Sp
\&\f(CW\*(C`BEGIN\*(C'\fR and \f(CW\*(C`END\*(C'\fR blocks may be used to capture control before or after
the implicit loop, just as in \fIawk\fR.
.IP "\fB\-s\fR" 5
.IX Xref "-s"
.IX Item "-s"
enables rudimentary switch parsing for switches on the command
line after the program name but before any filename arguments (or before
an argument of \fB\-\-\fR).  Any switch found there is removed from \f(CW@ARGV\fR and sets the
corresponding variable in the Perl program.  The following program
prints \*(L"1\*(R" if the program is invoked with a \fB\-xyz\fR switch, and \*(L"abc\*(R"
if it is invoked with \fB\-xyz=abc\fR.
.Sp
.Vb 2
\&    #!/usr/bin/perl \-s
\&    if ($xyz) { print "$xyz\en" }
.Ve
.Sp
Do note that a switch like \fB\-\-help\fR creates the variable \f(CW\*(C`${\-help}\*(C'\fR, which is not compliant
with \f(CW\*(C`use strict "refs"\*(C'\fR.  Also, when using this option on a script with
warnings enabled you may get a lot of spurious \*(L"used only once\*(R" warnings.
.IP "\fB\-S\fR" 5
.IX Xref "-S"
.IX Item "-S"
makes Perl use the \s-1PATH\s0 environment variable to search for the
program unless the name of the program contains path separators.
.Sp
On some platforms, this also makes Perl append suffixes to the
filename while searching for it.  For example, on Win32 platforms,
the \*(L".bat\*(R" and \*(L".cmd\*(R" 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 \f(CW\*(C`DEBUGGING\*(C'\fR turned
on, using the \fB\-Dp\fR switch to Perl shows how the search progresses.
.Sp
Typically this is used to emulate \f(CW\*(C`#!\*(C'\fR startup on platforms that don't
support \f(CW\*(C`#!\*(C'\fR.  It's also convenient when debugging a script that uses \f(CW\*(C`#!\*(C'\fR,
and is thus normally found by the shell's \f(CW$PATH\fR search mechanism.
.Sp
This example works on many platforms that have a shell compatible with
Bourne shell:
.Sp
.Vb 3
\&    #!/usr/bin/perl
\&    eval \*(Aqexec /usr/bin/perl \-wS $0 ${1+"$@"}\*(Aq
\&            if $running_under_some_shell;
.Ve
.Sp
The system ignores the first line and feeds the program to \fI/bin/sh\fR,
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 \f(CW$0\fR doesn't always
contain the full pathname, so the \fB\-S\fR tells Perl to search for the
program if necessary.  After Perl locates the program, it parses the
lines and ignores them because the variable \f(CW$running_under_some_shell\fR
is never true.  If the program will be interpreted by csh, you will need
to replace \f(CW\*(C`${1+"$@"}\*(C'\fR with \f(CW$*\fR, even though that doesn't understand
embedded spaces (and such) in the argument list.  To start up \fIsh\fR rather
than \fIcsh\fR, some systems may have to replace the \f(CW\*(C`#!\*(C'\fR 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 \fIcsh\fR, \fIsh\fR, or Perl, such as the following:
.Sp
.Vb 3
\&        eval \*(Aq(exit $?0)\*(Aq && eval \*(Aqexec perl \-wS $0 ${1+"$@"}\*(Aq
\&        & eval \*(Aqexec /usr/bin/perl \-wS $0 $argv:q\*(Aq
\&                if $running_under_some_shell;
.Ve
.Sp
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.
.Sp
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 \s-1PATH\s0.  On Unix platforms, the
program will be searched for strictly on the \s-1PATH\s0.
.IP "\fB\-t\fR" 5
.IX Xref "-t"
.IX Item "-t"
Like \fB\-T\fR, but taint checks will issue warnings rather than fatal
errors.  These warnings can now be controlled normally with \f(CW\*(C`no warnings
qw(taint)\*(C'\fR.
.Sp
\&\fBNote: This is not a substitute for \f(CB\*(C`\-T\*(C'\fB!\fR This is meant to be
used \fIonly\fR 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 \fB\-T\fR.
.IP "\fB\-T\fR" 5
.IX Xref "-T"
.IX Item "-T"
turns on \*(L"taint\*(R" 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 \s-1CGI\s0
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 \f(CW\*(C`#!\*(C'\fR line for systems which support
that construct.
.IP "\fB\-u\fR" 5
.IX Xref "-u"
.IX Item "-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 \fIundump\fR program (not supplied).
This speeds startup at the expense of some disk space (which you
can minimize by stripping the executable).  (Still, a \*(L"hello world\*(R"
executable comes out to about 200K on my machine.)  If you want to
execute a portion of your program before dumping, use the \fIdump()\fR
operator instead.  Note: availability of \fIundump\fR is platform
specific and may not be available for a specific port of Perl.
.IP "\fB\-U\fR" 5
.IX Xref "-U"
.IX Item "-U"
allows Perl to do unsafe operations.  Currently the only \*(L"unsafe\*(R"
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
\&\fIgenerate\fR the taint-check warnings.
.IP "\fB\-v\fR" 5
.IX Xref "-v"
.IX Item "-v"
prints the version and patchlevel of your perl executable.
.IP "\fB\-V\fR" 5
.IX Xref "-V"
.IX Item "-V"
prints summary of the major perl configuration values and the current
values of \f(CW@INC\fR.
.IP "\fB\-V:\fR\fIconfigvar\fR" 5
.IX Item "-V:configvar"
Prints to \s-1STDOUT\s0 the value of the named configuration variable(s),
with multiples when your \f(CW\*(C`\f(CIconfigvar\f(CW\*(C'\fR argument looks like a regex (has
non-letters).  For example:
.Sp
.Vb 12
\&    $ perl \-V:libc
\&        libc=\*(Aq/lib/libc\-2.2.4.so\*(Aq;
\&    $ perl \-V:lib.
\&        libs=\*(Aq\-lnsl \-lgdbm \-ldb \-ldl \-lm \-lcrypt \-lutil \-lc\*(Aq;
\&        libc=\*(Aq/lib/libc\-2.2.4.so\*(Aq;
\&    $ perl \-V:lib.*
\&        libpth=\*(Aq/usr/local/lib /lib /usr/lib\*(Aq;
\&        libs=\*(Aq\-lnsl \-lgdbm \-ldb \-ldl \-lm \-lcrypt \-lutil \-lc\*(Aq;
\&        lib_ext=\*(Aq.a\*(Aq;
\&        libc=\*(Aq/lib/libc\-2.2.4.so\*(Aq;
\&        libperl=\*(Aqlibperl.a\*(Aq;
\&        ....
.Ve
.Sp
Additionally, extra colons can be used to control formatting.  A
trailing colon suppresses the linefeed and terminator \*(L";\*(R", allowing
you to embed queries into shell commands.  (mnemonic: \s-1PATH\s0 separator
\&\*(L":\*(R".)
.Sp
.Vb 2
\&    $ echo "compression\-vars: " \`perl \-V:z.*: \` " are here !"
\&    compression\-vars:  zcat=\*(Aq\*(Aq zip=\*(Aqzip\*(Aq  are here !
.Ve
.Sp
A leading colon removes the \*(L"name=\*(R" part of the response, this allows
you to map to the name you need.  (mnemonic: empty label)
.Sp
.Vb 2
\&    $ echo "goodvfork="\`./perl \-Ilib \-V::usevfork\`
\&    goodvfork=false;
.Ve
.Sp
Leading and trailing colons can be used together if you need
positional parameter values without the names.  Note that in the case
below, the \f(CW\*(C`PERL_API\*(C'\fR params are returned in alphabetical order.
.Sp
.Vb 2
\&    $ echo building_on \`perl \-V::osname: \-V::PERL_API_.*:\` now
\&    building_on \*(Aqlinux\*(Aq \*(Aq5\*(Aq \*(Aq1\*(Aq \*(Aq9\*(Aq now
.Ve
.IP "\fB\-w\fR" 5
.IX Xref "-w"
.IX Item "-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 \fIlook\fR like numbers;
using an array as though it were a scalar; if your subroutines
recurse more than 100 deep; and innumerable other things.
.Sp
This switch really just enables the global \f(CW$^W\fR variable; normally,
the lexically scoped \f(CW\*(C`use warnings\*(C'\fR pragma is preferred. You
can disable or promote into fatal errors specific warnings using
\&\f(CW\*(C`_\|_WARN_\|_\*(C'\fR hooks, as described in perlvar and \*(L"warn\*(R" in perlfunc.
See also perldiag and perltrap.  A fine-grained warning
facility is also available if you want to manipulate entire classes
of warnings; see warnings or perllexwarn.
.IP "\fB\-W\fR" 5
.IX Xref "-W"
.IX Item "-W"
Enables all warnings regardless of \f(CW\*(C`no warnings\*(C'\fR or \f(CW$^W\fR.
See perllexwarn.
.IP "\fB\-X\fR" 5
.IX Xref "-X"
.IX Item "-X"
Disables all warnings regardless of \f(CW\*(C`use warnings\*(C'\fR or \f(CW$^W\fR.
See perllexwarn.
.IP "\fB\-x\fR" 5
.IX Xref "-x"
.IX Item "-x"
.PD 0
.IP "\fB\-x\fR\fIdirectory\fR" 5
.IX Item "-xdirectory"
.PD
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 \f(CW\*(C`#!\*(C'\fR and contains the
string \*(L"perl\*(R".  Any meaningful switches on that line will be applied.
.Sp
All references to line numbers by the program (warnings, errors, ...)
will treat the \f(CW\*(C`#!\*(C'\fR 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 \f(CW\*(C`#line\*(C'\fR directive.
(See \*(L"Plain Old Comments (Not!)\*(R" in perlsyn)
.Sp
If a directory name is specified, Perl will switch to that directory
before running the program.  The \fB\-x\fR switch controls only the
disposal of leading garbage.  The program must be terminated with
\&\f(CW\*(C`_\|_END_\|_\*(C'\fR if there is trailing garbage to be ignored;  the program
can process any or all of the trailing garbage via the \f(CW\*(C`DATA\*(C'\fR filehandle
if desired.
.Sp
The directory, if specified, must appear immediately following the \fB\-x\fR
with no intervening whitespace.
.SH "ENVIRONMENT"
.IX Xref "perl, environment variables"
.IX Header "ENVIRONMENT"
.IP "\s-1HOME\s0" 12
.IX Xref "HOME"
.IX Item "HOME"
Used if \f(CW\*(C`chdir\*(C'\fR has no argument.
.IP "\s-1LOGDIR\s0" 12
.IX Xref "LOGDIR"
.IX Item "LOGDIR"
Used if \f(CW\*(C`chdir\*(C'\fR has no argument and \s-1HOME\s0 is not set.
.IP "\s-1PATH\s0" 12
.IX Xref "PATH"
.IX Item "PATH"
Used in executing subprocesses, and in finding the program if \fB\-S\fR is
used.
.IP "\s-1PERL5LIB\s0" 12
.IX Xref "PERL5LIB"
.IX Item "PERL5LIB"
A 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, with this lookup
done at interpreter startup time.
.Sp
If \s-1PERL5LIB\s0 is not defined, \s-1PERLLIB\s0 is used.  Directories are separated
(like in \s-1PATH\s0) by a colon on Unixish platforms and by a semicolon on
Windows (the proper path separator being given by the command \f(CW\*(C`perl
\&\-V:\f(CIpath_sep\f(CW\*(C'\fR).
.Sp
When running taint checks, either because the program was running setuid or
setgid, or the \fB\-T\fR or \fB\-t\fR switch was specified, neither \s-1PERL5LIB\s0 nor
\&\s-1PERLLIB\s0 is consulted. The program should instead say:
.Sp
.Vb 1
\&    use lib "/my/directory";
.Ve
.IP "\s-1PERL5OPT\s0" 12
.IX Xref "PERL5OPT"
.IX Item "PERL5OPT"
Command-line options (switches).  Switches in this variable are treated
as if they were on every Perl command line.  Only the \fB\-[CDIMUdmtwW]\fR
switches are allowed.  When running taint checks (either because the
program was running setuid or setgid, or because the \fB\-T\fR or \fB\-t\fR
switch was used), this variable is ignored.  If \s-1PERL5OPT\s0 begins with
\&\fB\- T\fR, tainting will be enabled and subsequent options ignored.  If
\&\s-1PERL5OPT\s0 begins with \fB\-t\fR, tainting will be enabled, a writable dot
removed from \f(CW@INC\fR, and subsequent options honored.
.IP "\s-1PERLIO\s0" 12
.IX Xref "PERLIO"
.IX Item "PERLIO"
A space (or colon) separated list of PerlIO layers. If perl is built
to use PerlIO system for \s-1IO\s0 (the default) these layers affect Perl's \s-1IO\s0.
.Sp
It is conventional to start layer names with a colon (for example, \f(CW\*(C`:perlio\*(C'\fR) to
emphasize their similarity to variable \*(L"attributes\*(R". But the code that parses
layer specification strings,  which is also used to decode the \s-1PERLIO\s0
environment variable, treats the colon as a separator.
.Sp
An unset or empty \s-1PERLIO\s0 is equivalent to the default set of layers for
your platform; for example, \f(CW\*(C`:unix:perlio\*(C'\fR on Unix-like systems
and \f(CW\*(C`:unix:crlf\*(C'\fR on Windows and other DOS-like systems.
.Sp
The list becomes the default for \fIall\fR Perl's \s-1IO\s0. Consequently only built-in
layers can appear in this list, as external layers (such as \f(CW\*(C`:encoding()\*(C'\fR) need
\&\s-1IO\s0 in  order to load them!. See \*(L"open pragma\*(R" for how to add external
encodings as defaults.
.Sp
Layers it makes sense to include in the \s-1PERLIO\s0 environment
variable are briefly summarized below. For more details see PerlIO.
.RS 12
.IP ":bytes" 8
.IX Xref ":bytes"
.IX Item ":bytes"
A pseudolayer that turns the \f(CW\*(C`:utf8\*(C'\fR flag \fIoff\fR for the layer below;
unlikely to be useful on its own in the global \s-1PERLIO\s0 environment variable.
You perhaps were thinking of \f(CW\*(C`:crlf:bytes\*(C'\fR or \f(CW\*(C`:perlio:bytes\*(C'\fR.
.IP ":crlf" 8
.IX Xref ":crlf"
.IX Item ":crlf"
A layer which does \s-1CRLF\s0 to \f(CW"\en"\fR translation distinguishing \*(L"text\*(R" and
\&\*(L"binary\*(R" files in the manner of MS-DOS and similar operating systems.
(It currently does \fInot\fR mimic MS-DOS as far as treating of Control-Z
as being an end-of-file marker.)
.IP ":mmap" 8
.IX Xref ":mmap"
.IX Item ":mmap"
A layer that implements \*(L"reading\*(R" of files by using \fImmap\fR(2) to
make an entire file appear in the process's address space, and then
using that as PerlIO's \*(L"buffer\*(R".
.IP ":perlio" 8
.IX Xref ":perlio"
.IX Item ":perlio"
This is a re-implementation of stdio-like buffering written as a
PerlIO layer.  As such it will call whatever layer is below it for
its operations, typically \f(CW\*(C`:unix\*(C'\fR.
.IP ":pop" 8
.IX Xref ":pop"
.IX Item ":pop"
An experimental pseudolayer that removes the topmost layer.
Use with the same care as is reserved for nitroglycerine.
.IP ":raw" 8
.IX Xref ":raw"
.IX Item ":raw"
A pseudolayer that manipulates other layers.  Applying the \f(CW\*(C`:raw\*(C'\fR
layer is equivalent to calling \f(CW\*(C`binmode($fh)\*(C'\fR.  It makes the stream
pass each byte as-is without translation.  In particular, both \s-1CRLF\s0
translation and intuiting \f(CW\*(C`:utf8\*(C'\fR from the locale are disabled.
.Sp
Unlike in earlier versions of Perl, \f(CW\*(C`:raw\*(C'\fR is \fInot\fR
just the inverse of \f(CW\*(C`:crlf\*(C'\fR: other layers which would affect the
binary nature of the stream are also removed or disabled.
.IP ":stdio" 8
.IX Xref ":stdio"
.IX Item ":stdio"
This layer provides a PerlIO interface by wrapping system's \s-1ANSI\s0 C \*(L"stdio\*(R"
library calls. The layer provides both buffering and \s-1IO\s0.
Note that the \f(CW\*(C`:stdio\*(C'\fR layer does \fInot\fR do \s-1CRLF\s0 translation even if that
is the platform's normal behaviour. You will need a \f(CW\*(C`:crlf\*(C'\fR layer above it
to do that.
.IP ":unix" 8
.IX Xref ":unix"
.IX Item ":unix"
Low-level layer that calls \f(CW\*(C`read\*(C'\fR, \f(CW\*(C`write\*(C'\fR, \f(CW\*(C`lseek\*(C'\fR, etc.
.IP ":utf8" 8
.IX Xref ":utf8"
.IX Item ":utf8"
A pseudolayer that enables a flag in the layer below to tell Perl
that output should be in utf8 and that input should be regarded as
already in valid utf8 form. \fB\s-1WARNING:\s0 It does not check for validity and as such
should be handled with extreme caution for input, because security violations
can occur with non-shortest \s-1UTF\-8\s0 encodings, etc.\fR Generally \f(CW\*(C`:encoding(utf8)\*(C'\fR is
the best option when reading \s-1UTF\-8\s0 encoded data.
.IP ":win32" 8
.IX Xref ":win32"
.IX Item ":win32"
On Win32 platforms this \fIexperimental\fR layer uses native \*(L"handle\*(R" \s-1IO\s0
rather than a Unix-like numeric file descriptor layer. Known to be
buggy in this release (5.14).
.RE
.RS 12
.Sp
The default set of layers should give acceptable results on all platforms
.Sp
For Unix platforms that will be the equivalent of \*(L"unix perlio\*(R" or \*(L"stdio\*(R".
Configure is set up to prefer the \*(L"stdio\*(R" implementation if the system's library
provides for fast access to the buffer; otherwise, it uses the \*(L"unix perlio\*(R"
implementation.
.Sp
On Win32 the default in this release (5.14) is \*(L"unix crlf\*(R". Win32's \*(L"stdio\*(R"
has a number of bugs/mis\-features for Perl \s-1IO\s0 which are somewhat depending
on the version and vendor of the C compiler. Using our own \f(CW\*(C`crlf\*(C'\fR layer as
the buffer avoids those issues and makes things more uniform.  The \f(CW\*(C`crlf\*(C'\fR
layer provides \s-1CRLF\s0 conversion as well as buffering.
.Sp
This release (5.14) uses \f(CW\*(C`unix\*(C'\fR as the bottom layer on Win32, and so still
uses the C compiler's numeric file descriptor routines. There is an
experimental native \f(CW\*(C`win32\*(C'\fR layer, which is expected to be enhanced and
should eventually become the default under Win32.
.Sp
The \s-1PERLIO\s0 environment variable is completely ignored when Perl
is run in taint mode.
.RE
.IP "\s-1PERLIO_DEBUG\s0" 12
.IX Xref "PERLIO_DEBUG"
.IX Item "PERLIO_DEBUG"
If set to the name of a file or device, certain operations of PerlIO
subsystem will be logged to that file, which is opened in append mode
Typical uses are in Unix:
.Sp
.Vb 1
\&   % env PERLIO_DEBUG=/dev/tty perl script ...
.Ve
.Sp
and under Win32, the approximately equivalent:
.Sp
.Vb 2
\&   > set PERLIO_DEBUG=CON
\&   perl script ...
.Ve
.Sp
This functionality is disabled for setuid scripts and for scripts run
with \fB\-T\fR.
.IP "\s-1PERLLIB\s0" 12
.IX Xref "PERLLIB"
.IX Item "PERLLIB"
A list of directories in which to look for Perl library
files before looking in the standard library and the current directory.
If \s-1PERL5LIB\s0 is defined, \s-1PERLLIB\s0 is not used.
.Sp
The \s-1PERLLIB\s0 environment variable is completely ignored when Perl
is run in taint mode.
.IP "\s-1PERL5DB\s0" 12
.IX Xref "PERL5DB"
.IX Item "PERL5DB"
The command used to load the debugger code.  The default is:
.Sp
.Vb 1
\&        BEGIN { require "perl5db.pl" }
.Ve
.Sp
The \s-1PERL5DB\s0 environment variable is only used when Perl is started with
a bare \fB\-d\fR switch.
.IP "\s-1PERL5DB_THREADED\s0" 12
.IX Xref "PERL5DB_THREADED"
.IX Item "PERL5DB_THREADED"
If set to a true value, indicates to the debugger that the code being
debugged uses threads.
.IP "\s-1PERL5SHELL\s0 (specific to the Win32 port)" 12
.IX Xref "PERL5SHELL"
.IX Item "PERL5SHELL (specific to the Win32 port)"
On Win32 ports only, may be set to an alternative shell that Perl must use
internally for executing \*(L"backtick\*(R" commands or \fIsystem()\fR.  Default is
\&\f(CW\*(C`cmd.exe /x/d/c\*(C'\fR on WindowsNT and \f(CW\*(C`command.com /c\*(C'\fR on Windows95.  The
value is considered space-separated.  Precede any character that
needs to be protected, like a space or backslash, with another backslash.
.Sp
Note that Perl doesn't use \s-1COMSPEC\s0 for this purpose because
\&\s-1COMSPEC\s0 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 \s-1COMSPEC\s0 to such a shell may
interfere with the proper functioning of other programs (which usually
look in \s-1COMSPEC\s0 to find a shell fit for interactive use).
.Sp
Before Perl 5.10.0 and 5.8.8, \s-1PERL5SHELL\s0 was not taint checked
when running external commands.  It is recommended that
you explicitly set (or delete) \f(CW$ENV{PERL5SHELL}\fR when running
in taint mode under Windows.
.IP "\s-1PERL_ALLOW_NON_IFS_LSP\s0 (specific to the Win32 port)" 12
.IX Xref "PERL_ALLOW_NON_IFS_LSP"
.IX Item "PERL_ALLOW_NON_IFS_LSP (specific to the Win32 port)"
Set to 1 to allow the use of non-IFS compatible LSPs (Layered Service Providers).
Perl normally searches for an IFS-compatible \s-1LSP\s0 because this is required
for its emulation of Windows sockets as real filehandles.  However, this may
cause problems if you have a firewall such as \fIMcAfee Guardian\fR, which requires
that all applications use its \s-1LSP\s0 but which is not IFS-compatible, because clearly
Perl will normally avoid using such an \s-1LSP\s0.
.Sp
Setting this environment variable to 1 means that Perl will simply use the
first suitable \s-1LSP\s0 enumerated in the catalog, which keeps \fIMcAfee Guardian\fR
happy\*(--and in that particular case Perl still works too because \fIMcAfee
Guardian\fR's \s-1LSP\s0 actually plays other games which allow applications
requiring \s-1IFS\s0 compatibility to work.
.IP "\s-1PERL_DEBUG_MSTATS\s0" 12
.IX Xref "PERL_DEBUG_MSTATS"
.IX Item "PERL_DEBUG_MSTATS"
Relevant only if Perl is compiled with the \f(CW\*(C`malloc\*(C'\fR included with the Perl
distribution; that is, if \f(CW\*(C`perl \-V:d_mymalloc\*(C'\fR is \*(L"define\*(R".
.Sp
If set, this dumps out memory statistics after execution.  If set
to an integer greater than one, also dumps out memory statistics
after compilation.
.IP "\s-1PERL_DESTRUCT_LEVEL\s0" 12
.IX Xref "PERL_DESTRUCT_LEVEL"
.IX Item "PERL_DESTRUCT_LEVEL"
Relevant only if your Perl executable was built with \fB\-DDEBUGGING\fR,
this controls the behaviour of global destruction of objects and other
references.  See \*(L"\s-1PERL_DESTRUCT_LEVEL\s0\*(R" in perlhacktips for more information.
.IP "\s-1PERL_DL_NONLAZY\s0" 12
.IX Xref "PERL_DL_NONLAZY"
.IX Item "PERL_DL_NONLAZY"
Set to \f(CW"1"\fR to have Perl resolve \fIall\fR undefined symbols when it loads
a dynamic library.  The default behaviour is to resolve symbols when
they are used.  Setting this variable is useful during testing of
extensions, as it ensures that you get an error on misspelled function
names even if the test suite doesn't call them.
.IP "\s-1PERL_ENCODING\s0" 12
.IX Xref "PERL_ENCODING"
.IX Item "PERL_ENCODING"
If using the \f(CW\*(C`use encoding\*(C'\fR pragma without an explicit encoding name, the
\&\s-1PERL_ENCODING\s0 environment variable is consulted for an encoding name.
.IP "\s-1PERL_HASH_SEED\s0" 12
.IX Xref "PERL_HASH_SEED"
.IX Item "PERL_HASH_SEED"
(Since Perl 5.8.1.)  Used to randomize Perl's internal hash function.
To emulate the pre\-5.8.1 behaviour, set to an integer; \f(CW"0"\fR means
exactly the same order as in 5.8.0.  \*(L"Pre\-5.8.1\*(R" means, among other
things, that hash keys will always have the same ordering between
different runs of Perl.
.Sp
Most hashes by default return elements in the same order as in Perl 5.8.0.
On a hash by hash basis, if pathological data is detected during a hash
key insertion, then that hash will switch to an alternative random hash
seed.
.Sp
The default behaviour is to randomize unless the \s-1PERL_HASH_SEED\s0 is set.
If Perl has been compiled with \fB\-DUSE_HASH_SEED_EXPLICIT\fR, the default
behaviour is \fInot\fR to randomize unless the \s-1PERL_HASH_SEED\s0 is set.
.Sp
If \s-1PERL_HASH_SEED\s0 is unset or set to a non-numeric string, Perl uses
the pseudorandom seed supplied by the operating system and libraries.
.Sp
\&\fB\s-1PLEASE\s0 \s-1NOTE:\s0 The hash seed is sensitive information\fR. Hashes are
randomized to protect against local and remote attacks against Perl
code. By manually setting a seed, this protection may be partially or
completely lost.
.Sp
See \*(L"Algorithmic Complexity Attacks\*(R" in perlsec and
\&\*(L"\s-1PERL_HASH_SEED_DEBUG\s0\*(R" for more information.
.IP "\s-1PERL_HASH_SEED_DEBUG\s0" 12
.IX Xref "PERL_HASH_SEED_DEBUG"
.IX Item "PERL_HASH_SEED_DEBUG"
(Since Perl 5.8.1.)  Set to \f(CW"1"\fR to display (to \s-1STDERR\s0) the value of
the hash seed at the beginning of execution.  This, combined with
\&\*(L"\s-1PERL_HASH_SEED\s0\*(R" is intended to aid in debugging nondeterministic
behaviour caused by hash randomization.
.Sp
\&\fBNote that the hash seed is sensitive information\fR: by knowing it, one
can craft a denial-of-service attack against Perl code, even remotely;
see \*(L"Algorithmic Complexity Attacks\*(R" in perlsec for more information.
\&\fBDo not disclose the hash seed\fR to people who don't need to know it.
See also \fIhash_seed()\fR in Hash::Util.
.IP "\s-1PERL_MEM_LOG\s0" 12
.IX Xref "PERL_MEM_LOG"
.IX Item "PERL_MEM_LOG"
If your Perl was configured with \fB\-Accflags=\-DPERL_MEM_LOG\fR, setting
the environment variable \f(CW\*(C`PERL_MEM_LOG\*(C'\fR enables logging debug
messages. The value has the form \f(CW\*(C`<\f(CInumber\f(CW>[m][s][t]\*(C'\fR, where
\&\f(CW\*(C`\f(CInumber\f(CW\*(C'\fR is the file descriptor number you want to write to (2 is
default), and the combination of letters specifies that you want
information about (m)emory and/or (s)v, optionally with
(t)imestamps. For example, \f(CW\*(C`PERL_MEM_LOG=1mst\*(C'\fR logs all
information to stdout. You can write to other opened file descriptors
in a variety of ways:
.Sp
.Vb 1
\&  $ 3>foo3 PERL_MEM_LOG=3m perl ...
.Ve
.IP "\s-1PERL_ROOT\s0 (specific to the \s-1VMS\s0 port)" 12
.IX Xref "PERL_ROOT"
.IX Item "PERL_ROOT (specific to the VMS port)"
A translation-concealed rooted logical name that contains Perl and the
logical device for the \f(CW@INC\fR path on \s-1VMS\s0 only.  Other logical names that
affect Perl on \s-1VMS\s0 include \s-1PERLSHR\s0, \s-1PERL_ENV_TABLES\s0, and
\&\s-1SYS$TIMEZONE_DIFFERENTIAL\s0, but are optional and discussed further in
perlvms and in \fI\s-1README\s0.vms\fR in the Perl source distribution.
.IP "\s-1PERL_SIGNALS\s0" 12
.IX Xref "PERL_SIGNALS"
.IX Item "PERL_SIGNALS"
Available in Perls 5.8.1 and later.  If set to \f(CW"unsafe"\fR, the pre\-Perl\-5.8.0
signal behaviour (which is immediate but unsafe) is restored.  If set
to \f(CW\*(C`safe\*(C'\fR, then safe (but deferred) signals are used.  See
\&\*(L"Deferred Signals (Safe Signals)\*(R" in perlipc.
.IP "\s-1PERL_UNICODE\s0" 12
.IX Xref "PERL_UNICODE"
.IX Item "PERL_UNICODE"
Equivalent to the \fB\-C\fR command-line switch.  Note that this is not
a boolean variable. Setting this to \f(CW"1"\fR is not the right way to
\&\*(L"enable Unicode\*(R" (whatever that would mean).  You can use \f(CW"0"\fR to
\&\*(L"disable Unicode\*(R", though (or alternatively unset \s-1PERL_UNICODE\s0 in
your shell before starting Perl).  See the description of the \fB\-C\fR
switch for more information.
.IP "\s-1SYS$LOGIN\s0 (specific to the \s-1VMS\s0 port)" 12
.IX Xref "SYS$LOGIN"
.IX Item "SYS$LOGIN (specific to the VMS port)"
Used if chdir has no argument and \s-1HOME\s0 and \s-1LOGDIR\s0 are not set.
.PP
Perl also has environment variables that control how Perl handles data
specific to particular natural languages; see perllocale.
.PP
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.
.PP
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:
.PP
.Vb 3
\&    $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)};
.Ve
