extra code in pp_concat, Take 2
[p5sagit/p5-mst-13.2.git] / pod / perlrun.pod
CommitLineData
a0d0e21e 1=head1 NAME
2
3perlrun - how to execute the Perl interpreter
4
5=head1 SYNOPSIS
6
672fde27 7B<perl> S<[ B<-sTtuUWX> ]>
e0ebc809 8 S<[ B<-hv> ] [ B<-V>[:I<configvar>] ]>
2cbb2ee1 9 S<[ B<-cw> ] [ B<-d>[B<t>][:I<debugger>] ] [ B<-D>[I<number/list>] ]>
f2095865 10 S<[ B<-pna> ] [ B<-F>I<pattern> ] [ B<-l>[I<octal>] ] [ B<-0>[I<octal/hexadecimal>] ]>
e0ebc809 11 S<[ B<-I>I<dir> ] [ B<-m>[B<->]I<module> ] [ B<-M>[B<->]I<'module...'> ]>
12 S<[ B<-P> ]>
13 S<[ B<-S> ]>
14 S<[ B<-x>[I<dir>] ]>
15 S<[ B<-i>[I<extension>] ]>
16 S<[ B<-e> I<'command'> ] [ B<--> ] [ I<programfile> ] [ I<argument> ]...>
702815ca 17 S<[ B<-A [I<assertions>] >]>
18 S<[ B<-C [I<number/list>] >]>
a0d0e21e 19
20=head1 DESCRIPTION
21
19799a22 22The normal way to run a Perl program is by making it directly
23executable, or else by passing the name of the source file as an
24argument on the command line. (An interactive Perl environment
25is also possible--see L<perldebug> for details on how to do that.)
26Upon startup, Perl looks for your program in one of the following
a0d0e21e 27places:
28
29=over 4
30
31=item 1.
32
33Specified line by line via B<-e> switches on the command line.
34
35=item 2.
36
37Contained in the file specified by the first filename on the command line.
a3cb178b 38(Note that systems supporting the #! notation invoke interpreters this
39way. See L<Location of Perl>.)
a0d0e21e 40
41=item 3.
42
5f05dabc 43Passed in implicitly via standard input. This works only if there are
19799a22 44no filename arguments--to pass arguments to a STDIN-read program you
45must explicitly specify a "-" for the program name.
a0d0e21e 46
47=back
48
49With methods 2 and 3, Perl starts parsing the input file from the
50beginning, unless you've specified a B<-x> switch, in which case it
51scans for the first line starting with #! and containing the word
19799a22 52"perl", and starts there instead. This is useful for running a program
a0d0e21e 53embedded in a larger message. (In this case you would indicate the end
19799a22 54of the program using the C<__END__> token.)
a0d0e21e 55
5f05dabc 56The #! line is always examined for switches as the line is being
57parsed. Thus, if you're on a machine that allows only one argument
58with the #! line, or worse, doesn't even recognize the #! line, you
59still can get consistent switch behavior regardless of how Perl was
19799a22 60invoked, even if B<-x> was used to find the beginning of the program.
61
62Because historically some operating systems silently chopped off
63kernel interpretation of the #! line after 32 characters, some
64switches may be passed in on the command line, and some may not;
65you could even get a "-" without its letter, if you're not careful.
66You probably want to make sure that all your switches fall either
67before or after that 32-character boundary. Most switches don't
68actually care if they're processed redundantly, but getting a "-"
69instead of a complete switch could cause Perl to try to execute
70standard input instead of your program. And a partial B<-I> switch
a0d0e21e 71could also cause odd results.
72
19799a22 73Some switches do care if they are processed twice, for instance
74combinations of B<-l> and B<-0>. Either put all the switches after
75the 32-character boundary (if applicable), or replace the use of
76B<-0>I<digits> by C<BEGIN{ $/ = "\0digits"; }>.
fb73857a 77
a0d0e21e 78Parsing of the #! switches starts wherever "perl" is mentioned in the line.
79The sequences "-*" and "- " are specifically ignored so that you could,
80if you were so inclined, say
81
82 #!/bin/sh -- # -*- perl -*- -p
19799a22 83 eval 'exec perl -wS $0 ${1+"$@"}'
5f05dabc 84 if $running_under_some_shell;
a0d0e21e 85
44a4342c 86to let Perl see the B<-p> switch.
19799a22 87
88A similar trick involves the B<env> program, if you have it.
89
90 #!/usr/bin/env perl
91
92The examples above use a relative path to the perl interpreter,
93getting whatever version is first in the user's path. If you want
94a specific version of Perl, say, perl5.005_57, you should place
95that directly in the #! line's path.
a0d0e21e 96
97If the #! line does not contain the word "perl", the program named after
98the #! is executed instead of the Perl interpreter. This is slightly
99bizarre, but it helps people on machines that don't do #!, because they
19799a22 100can tell a program that their SHELL is F</usr/bin/perl>, and Perl will then
a0d0e21e 101dispatch the program to the correct interpreter for them.
102
19799a22 103After locating your program, Perl compiles the entire program to an
a0d0e21e 104internal form. If there are any compilation errors, execution of the
19799a22 105program is not attempted. (This is unlike the typical shell script,
54310121 106which might run part-way through before finding a syntax error.)
a0d0e21e 107
19799a22 108If the program is syntactically correct, it is executed. If the program
a0d0e21e 109runs off the end without hitting an exit() or die() operator, an implicit
110C<exit(0)> is provided to indicate successful completion.
111
68dc0745 112=head2 #! and quoting on non-Unix systems
113
114Unix's #! technique can be simulated on other systems:
115
116=over 4
117
118=item OS/2
119
120Put
121
122 extproc perl -S -your_switches
123
19799a22 124as the first line in C<*.cmd> file (B<-S> due to a bug in cmd.exe's
68dc0745 125`extproc' handling).
126
54310121 127=item MS-DOS
68dc0745 128
19799a22 129Create a batch file to run your program, and codify it in
fd1adc71 130C<ALTERNATE_SHEBANG> (see the F<dosish.h> file in the source
68dc0745 131distribution for more information).
132
133=item Win95/NT
134
6c6a61e2 135The Win95/NT installation, when using the ActiveState installer for Perl,
c8db1d39 136will modify the Registry to associate the F<.pl> extension with the perl
6c6a61e2 137interpreter. If you install Perl by other means (including building from
138the sources), you may have to modify the Registry yourself. Note that
139this means you can no longer tell the difference between an executable
140Perl program and a Perl library file.
68dc0745 141
142=item Macintosh
143
8e30f651 144Under "Classic" MacOS, a perl program will have the appropriate Creator and
145Type, so that double-clicking them will invoke the MacPerl application.
146Under Mac OS X, clickable apps can be made from any C<#!> script using Wil
147Sanchez' DropScript utility: http://www.wsanchez.net/software/ .
68dc0745 148
bd3fa61c 149=item VMS
150
151Put
152
153 $ perl -mysw 'f$env("procedure")' 'p1' 'p2' 'p3' 'p4' 'p5' 'p6' 'p7' 'p8' !
154 $ exit++ + ++$status != 0 and $exit = $status = undef;
155
19799a22 156at the top of your program, where B<-mysw> are any command line switches you
157want to pass to Perl. You can now invoke the program directly, by saying
158C<perl program>, or as a DCL procedure, by saying C<@program> (or implicitly
159via F<DCL$PATH> by just using the name of the program).
bd3fa61c 160
161This incantation is a bit much to remember, but Perl will display it for
162you if you say C<perl "-V:startperl">.
163
68dc0745 164=back
165
166Command-interpreters on non-Unix systems have rather different ideas
167on quoting than Unix shells. You'll need to learn the special
168characters in your command-interpreter (C<*>, C<\> and C<"> are
169common) and how to protect whitespace and these characters to run
19799a22 170one-liners (see B<-e> below).
68dc0745 171
172On some systems, you may have to change single-quotes to double ones,
e6f03d26 173which you must I<not> do on Unix or Plan 9 systems. You might also
68dc0745 174have to change a single % to a %%.
175
176For example:
177
178 # Unix
179 perl -e 'print "Hello world\n"'
180
54310121 181 # MS-DOS, etc.
68dc0745 182 perl -e "print \"Hello world\n\""
183
54310121 184 # Macintosh
68dc0745 185 print "Hello world\n"
186 (then Run "Myscript" or Shift-Command-R)
187
188 # VMS
189 perl -e "print ""Hello world\n"""
190
19799a22 191The problem is that none of this is reliable: it depends on the
192command and it is entirely possible neither works. If B<4DOS> were
193the command shell, this would probably work better:
68dc0745 194
195 perl -e "print <Ctrl-x>"Hello world\n<Ctrl-x>""
196
19799a22 197B<CMD.EXE> in Windows NT slipped a lot of standard Unix functionality in
68dc0745 198when nobody was looking, but just try to find documentation for its
199quoting rules.
200
54310121 201Under the Macintosh, it depends which environment you are using. The MacPerl
68dc0745 202shell, or MPW, is much like Unix shells in its support for several
54310121 203quoting variants, except that it makes free use of the Macintosh's non-ASCII
68dc0745 204characters as control characters.
205
206There is no general solution to all of this. It's just a mess.
207
a3cb178b 208=head2 Location of Perl
209
210It may seem obvious to say, but Perl is useful only when users can
19799a22 211easily find it. When possible, it's good for both F</usr/bin/perl>
212and F</usr/local/bin/perl> to be symlinks to the actual binary. If
213that can't be done, system administrators are strongly encouraged
214to put (symlinks to) perl and its accompanying utilities into a
215directory typically found along a user's PATH, or in some other
216obvious and convenient place.
217
218In this documentation, C<#!/usr/bin/perl> on the first line of the program
219will stand in for whatever method works on your system. You are
220advised to use a specific path if you care about a specific version.
a3cb178b 221
19799a22 222 #!/usr/local/bin/perl5.00554
a3cb178b 223
19799a22 224or if you just want to be running at least version, place a statement
225like this at the top of your program:
a0d0e21e 226
19799a22 227 use 5.005_54;
a0d0e21e 228
19799a22 229=head2 Command Switches
230
231As with all standard commands, a single-character switch may be
232clustered with the following switch, if any.
233
234 #!/usr/bin/perl -spi.orig # same as -s -p -i.orig
a0d0e21e 235
236Switches include:
237
238=over 5
239
f2095865 240=item B<-0>[I<octal/hexadecimal>]
a0d0e21e 241
f2095865 242specifies the input record separator (C<$/>) as an octal or
243hexadecimal number. If there are no digits, the null character is the
244separator. Other switches may precede or follow the digits. For
245example, if you have a version of B<find> which can print filenames
246terminated by the null character, you can say this:
a0d0e21e 247
19799a22 248 find . -name '*.orig' -print0 | perl -n0e unlink
a0d0e21e 249
250The special value 00 will cause Perl to slurp files in paragraph mode.
5f05dabc 251The value 0777 will cause Perl to slurp files whole because there is no
f2095865 252legal byte with that value.
253
254If you want to specify any Unicode character, use the hexadecimal
255format: C<-0xHHH...>, where the C<H> are valid hexadecimal digits.
256(This means that you cannot use the C<-x> with a directory name that
257consists of hexadecimal digits.)
a0d0e21e 258
702815ca 259=item B<-A [I<assertions>]>
260
261Activates the assertions given after the switch as a comma-separated
262list of assertion names. If no assertion name is given, activates all
263assertions. See L<assertions>.
264
a0d0e21e 265=item B<-a>
266
267turns on autosplit mode when used with a B<-n> or B<-p>. An implicit
268split command to the @F array is done as the first thing inside the
269implicit while loop produced by the B<-n> or B<-p>.
270
271 perl -ane 'print pop(@F), "\n";'
272
273is equivalent to
274
275 while (<>) {
276 @F = split(' ');
277 print pop(@F), "\n";
278 }
279
280An alternate delimiter may be specified using B<-F>.
281
a05d7ebb 282=item B<-C [I<number/list>]>
46487f74 283
a05d7ebb 284The C<-C> flag controls some Unicode of the Perl Unicode features.
285
286As of 5.8.1, the C<-C> can be followed either by a number or a list
f3f8427d 287of option letters. The letters, their numeric values, and effects
8aa8f774 288are as follows; listing the letters is equal to summing the numbers.
9f21530f 289
73e12209 290 I 1 STDIN is assumed to be in UTF-8
291 O 2 STDOUT will be in UTF-8
292 E 4 STDERR will be in UTF-8
293 S 7 I + O + E
294 i 8 UTF-8 is the default PerlIO layer for input streams
295 o 16 UTF-8 is the default PerlIO layer for output streams
296 D 24 i + o
297 A 32 the @ARGV elements are expected to be strings encoded
298 in UTF-8
299 L 64 normally the "IOEioA" are unconditional,
300 the L makes them conditional on the locale environment
301 variables (the LC_ALL, LC_TYPE, and LANG, in the order
302 of decreasing precedence) -- if the variables indicate
303 UTF-8, then the selected "IOEioA" are in effect
9f21530f 304
305For example, C<-COE> and C<-C6> will both turn on UTF-8-ness on both
306STDOUT and STDERR. Repeating letters is just redundant, not cumulative
307nor toggling.
a05d7ebb 308
44505768 309The C<io> options mean that any subsequent open() (or similar I/O
310operations) will have the C<:utf8> PerlIO layer implicitly applied
311to them, in other words, UTF-8 is expected from any input stream,
312and UTF-8 is produced to any output stream. This is just the default,
313with explicit layers in open() and with binmode() one can manipulate
314streams as usual.
315
8aa8f774 316C<-C> on its own (not followed by any number or option list), or the
47427c4e 317empty string C<""> for the C<PERL_UNICODE> environment variable, has the
318same effect as C<-CSDL>. In other words, the standard I/O handles and
319the default C<open()> layer are UTF-8-fied B<but> only if the locale
320environment variables indicate a UTF-8 locale. This behaviour follows
321the I<implicit> (and problematic) UTF-8 behaviour of Perl 5.8.0.
a05d7ebb 322
47427c4e 323You can use C<-C0> (or C<"0"> for C<PERL_UNICODE>) to explicitly
5b4f334e 324disable all the above Unicode features.
fde18df1 325
8aa8f774 326The read-only magic variable C<${^UNICODE}> reflects the numeric value
ab9e1bb7 327of this setting. This is variable is set during Perl startup and is
328thereafter read-only. If you want runtime effects, use the three-arg
2307c6d0 329open() (see L<perlfunc/open>), the two-arg binmode() (see L<perlfunc/binmode>),
ab9e1bb7 330and the C<open> pragma (see L<open>).
fde18df1 331
332(In Perls earlier than 5.8.1 the C<-C> switch was a Win32-only switch
333that enabled the use of Unicode-aware "wide system call" Win32 APIs.
334This feature was practically unused, however, and the command line
335switch was therefore "recycled".)
46487f74 336
a0d0e21e 337=item B<-c>
338
19799a22 339causes Perl to check the syntax of the program and then exit without
7d30b5c4 340executing it. Actually, it I<will> execute C<BEGIN>, C<CHECK>, and
4f25aa18 341C<use> blocks, because these are considered as occurring outside the
342execution of your program. C<INIT> and C<END> blocks, however, will
343be skipped.
a0d0e21e 344
345=item B<-d>
346
2cbb2ee1 347=item B<-dt>
348
19799a22 349runs the program under the Perl debugger. See L<perldebug>.
2cbb2ee1 350If B<t> is specified, it indicates to the debugger that threads
351will be used in the code being debugged.
a0d0e21e 352
70c94a19 353=item B<-d:>I<foo[=bar,baz]>
3c81428c 354
2cbb2ee1 355=item B<-dt:>I<foo[=bar,baz]>
356
19799a22 357runs the program under the control of a debugging, profiling, or
358tracing module installed as Devel::foo. E.g., B<-d:DProf> executes
70c94a19 359the program using the Devel::DProf profiler. As with the B<-M>
360flag, options may be passed to the Devel::foo package where they
361will be received and interpreted by the Devel::foo::import routine.
362The comma-separated list of options must follow a C<=> character.
2cbb2ee1 363If B<t> is specified, it indicates to the debugger that threads
364will be used in the code being debugged.
70c94a19 365See L<perldebug>.
3c81428c 366
db2ba183 367=item B<-D>I<letters>
a0d0e21e 368
db2ba183 369=item B<-D>I<number>
a0d0e21e 370
19799a22 371sets debugging flags. To watch how it executes your program, use
db2ba183 372B<-Dtls>. (This works only if debugging is compiled into your
373Perl.) Another nice value is B<-Dx>, which lists your compiled
4197b13f 374syntax tree. And B<-Dr> displays compiled regular expressions;
44a4342c 375the format of the output is explained in L<perldebguts>.
4197b13f 376
377As an alternative, specify a number instead of list of letters (e.g.,
378B<-D14> is equivalent to B<-Dtls>):
a0d0e21e 379
9388183f 380 1 p Tokenizing and parsing (with v, displays parse stack)
3679267a 381 2 s Stack snapshots (with v, displays all stacks)
db2ba183 382 4 l Context (loop) stack processing
383 8 t Trace execution
384 16 o Method and overloading resolution
385 32 c String/numeric conversions
1045810a 386 64 P Print profiling info, preprocessor command for -P, source file input state
db2ba183 387 128 m Memory allocation
388 256 f Format processing
389 512 r Regular expression parsing and execution
390 1024 x Syntax tree dump
391 2048 u Tainting checks
7bab3ede 392 4096 (Obsolete, previously used for LEAKTEST)
db2ba183 393 8192 H Hash dump -- usurps values()
394 16384 X Scratchpad allocation
395 32768 D Cleaning up
8b73bbec 396 65536 S Thread synchronization
607df283 397 131072 T Tokenising
04932ac8 398 262144 R Include reference counts of dumped variables (eg when using -Ds)
1045810a 399 524288 J Do not s,t,P-debug (Jump over) opcodes within package DB
d6721266 400 1048576 v Verbose: use in conjunction with other flags
46187eeb 401 2097152 C Copy On Write
ecae49c0 402 4194304 A Consistency checks on internal structures
3679267a 403 8388608 q quiet - currently only suppresses the "EXECUTING" message
a0d0e21e 404
19799a22 405All these flags require B<-DDEBUGGING> when you compile the Perl
1045810a 406executable (but see L<Devel::Peek>, L<re> which may change this).
44a4342c 407See the F<INSTALL> file in the Perl source distribution
19799a22 408for how to do this. This flag is automatically set if you include B<-g>
8c52afec 409option when C<Configure> asks you about optimizer/debugger flags.
410
19799a22 411If you're just trying to get a print out of each line of Perl code
412as it executes, the way that C<sh -x> provides for shell scripts,
44a4342c 413you can't use Perl's B<-D> switch. Instead do this
19799a22 414
c406981e 415 # If you have "env" utility
fdac53cd 416 env PERLDB_OPTS="NonStop=1 AutoTrace=1 frame=2" perl -dS program
c406981e 417
19799a22 418 # Bourne shell syntax
419 $ PERLDB_OPTS="NonStop=1 AutoTrace=1 frame=2" perl -dS program
420
421 # csh syntax
422 % (setenv PERLDB_OPTS "NonStop=1 AutoTrace=1 frame=2"; perl -dS program)
423
424See L<perldebug> for details and variations.
425
a0d0e21e 426=item B<-e> I<commandline>
427
19799a22 428may be used to enter one line of program. If B<-e> is given, Perl
429will not look for a filename in the argument list. Multiple B<-e>
430commands may be given to build up a multi-line script. Make sure
431to use semicolons where you would in a normal program.
a0d0e21e 432
20ef40cf 433=item B<-f>
434
435Disable executing F<$Config{siteperl}/sitecustomize.pl> at
436startup.
437
438Perl can be built so that it by default will try to execute
439F<$Config{siteperl}/sitecustomize.pl> at startup. This is a hook that
440allows the sysadmin to customize how perl behaves. It can for
441instance be used to add entries to the @INC array to make perl find
442modules in non-standard locations.
443
e0ebc809 444=item B<-F>I<pattern>
a0d0e21e 445
e0ebc809 446specifies the pattern to split on if B<-a> is also in effect. The
5f05dabc 447pattern may be surrounded by C<//>, C<"">, or C<''>, otherwise it will be
e0ebc809 448put in single quotes.
a0d0e21e 449
e0ebc809 450=item B<-h>
451
452prints a summary of the options.
453
454=item B<-i>[I<extension>]
a0d0e21e 455
2d259d92 456specifies that files processed by the C<E<lt>E<gt>> construct are to be
457edited in-place. It does this by renaming the input file, opening the
458output file by the original name, and selecting that output file as the
459default for print() statements. The extension, if supplied, is used to
460modify the name of the old file to make a backup copy, following these
461rules:
462
463If no extension is supplied, no backup is made and the current file is
464overwritten.
465
19799a22 466If the extension doesn't contain a C<*>, then it is appended to the
467end of the current filename as a suffix. If the extension does
468contain one or more C<*> characters, then each C<*> is replaced
469with the current filename. In Perl terms, you could think of this
470as:
2d259d92 471
66606d78 472 ($backup = $extension) =~ s/\*/$file_name/g;
2d259d92 473
474This allows you to add a prefix to the backup file, instead of (or in
475addition to) a suffix:
476
ddffceb7 477 $ perl -pi'orig_*' -e 's/bar/baz/' fileA # backup to 'orig_fileA'
2d259d92 478
479Or even to place backup copies of the original files into another
480directory (provided the directory already exists):
481
ddffceb7 482 $ perl -pi'old/*.orig' -e 's/bar/baz/' fileA # backup to 'old/fileA.orig'
2d259d92 483
66606d78 484These sets of one-liners are equivalent:
485
486 $ perl -pi -e 's/bar/baz/' fileA # overwrite current file
ddffceb7 487 $ perl -pi'*' -e 's/bar/baz/' fileA # overwrite current file
66606d78 488
ddffceb7 489 $ perl -pi'.orig' -e 's/bar/baz/' fileA # backup to 'fileA.orig'
490 $ perl -pi'*.orig' -e 's/bar/baz/' fileA # backup to 'fileA.orig'
66606d78 491
2d259d92 492From the shell, saying
a0d0e21e 493
19799a22 494 $ perl -p -i.orig -e "s/foo/bar/; ... "
a0d0e21e 495
19799a22 496is the same as using the program:
a0d0e21e 497
19799a22 498 #!/usr/bin/perl -pi.orig
a0d0e21e 499 s/foo/bar/;
500
501which is equivalent to
502
503 #!/usr/bin/perl
19799a22 504 $extension = '.orig';
505 LINE: while (<>) {
a0d0e21e 506 if ($ARGV ne $oldargv) {
66606d78 507 if ($extension !~ /\*/) {
508 $backup = $ARGV . $extension;
509 }
510 else {
511 ($backup = $extension) =~ s/\*/$ARGV/g;
512 }
513 rename($ARGV, $backup);
a0d0e21e 514 open(ARGVOUT, ">$ARGV");
515 select(ARGVOUT);
516 $oldargv = $ARGV;
517 }
518 s/foo/bar/;
519 }
520 continue {
521 print; # this prints to original filename
522 }
523 select(STDOUT);
524
525except that the B<-i> form doesn't need to compare $ARGV to $oldargv to
526know when the filename has changed. It does, however, use ARGVOUT for
66606d78 527the selected filehandle. Note that STDOUT is restored as the default
528output filehandle after the loop.
529
530As shown above, Perl creates the backup file whether or not any output
531is actually changed. So this is just a fancy way to copy files:
532
cd2d1bac 533 $ perl -p -i'/some/file/path/*' -e 1 file1 file2 file3...
19799a22 534or
cd2d1bac 535 $ perl -p -i'.orig' -e 1 file1 file2 file3...
66606d78 536
537You can use C<eof> without parentheses to locate the end of each input
538file, in case you want to append to each file, or reset line numbering
539(see example in L<perlfunc/eof>).
540
541If, for a given file, Perl is unable to create the backup file as
542specified in the extension then it will skip that file and continue on
543with the next one (if it exists).
544
19799a22 545For a discussion of issues surrounding file permissions and B<-i>,
cea6626f 546see L<perlfaq5/Why does Perl let me delete read-only files? Why does -i clobber protected files? Isn't this a bug in Perl?>.
66606d78 547
548You cannot use B<-i> to create directories or to strip extensions from
549files.
a0d0e21e 550
19799a22 551Perl does not expand C<~> in filenames, which is good, since some
552folks use it for their backup files:
a0d0e21e 553
19799a22 554 $ perl -pi~ -e 's/foo/bar/' file1 file2 file3...
555
a66b22ca 556Note that because B<-i> renames or deletes the original file before
0cb0633f 557creating a new file of the same name, UNIX-style soft and hard links will
558not be preserved.
a66b22ca 559
19799a22 560Finally, the B<-i> switch does not impede execution when no
a2008d6d 561files are given on the command line. In this case, no backup is made
562(the original file cannot, of course, be determined) and processing
563proceeds from STDIN to STDOUT as might be expected.
564
a0d0e21e 565=item B<-I>I<directory>
566
e0ebc809 567Directories specified by B<-I> are prepended to the search path for
1fef88e7 568modules (C<@INC>), and also tells the C preprocessor where to search for
e0ebc809 569include files. The C preprocessor is invoked with B<-P>; by default it
570searches /usr/include and /usr/lib/perl.
a0d0e21e 571
e0ebc809 572=item B<-l>[I<octnum>]
a0d0e21e 573
19799a22 574enables automatic line-ending processing. It has two separate
575effects. First, it automatically chomps C<$/> (the input record
576separator) when used with B<-n> or B<-p>. Second, it assigns C<$\>
577(the output record separator) to have the value of I<octnum> so
578that any print statements will have that separator added back on.
579If I<octnum> is omitted, sets C<$\> to the current value of
580C<$/>. For instance, to trim lines to 80 columns:
a0d0e21e 581
582 perl -lpe 'substr($_, 80) = ""'
583
584Note that the assignment C<$\ = $/> is done when the switch is processed,
585so the input record separator can be different than the output record
586separator if the B<-l> switch is followed by a B<-0> switch:
587
588 gnufind / -print0 | perl -ln0e 'print "found $_" if -p'
589
1fef88e7 590This sets C<$\> to newline and then sets C<$/> to the null character.
a0d0e21e 591
e0ebc809 592=item B<-m>[B<->]I<module>
593
594=item B<-M>[B<->]I<module>
c07a80fd 595
e0ebc809 596=item B<-M>[B<->]I<'module ...'>
597
598=item B<-[mM]>[B<->]I<module=arg[,arg]...>
3c81428c 599
19799a22 600B<-m>I<module> executes C<use> I<module> C<();> before executing your
601program.
3c81428c 602
19799a22 603B<-M>I<module> executes C<use> I<module> C<;> before executing your
604program. You can use quotes to add extra code after the module name,
605e.g., C<'-Mmodule qw(foo bar)'>.
3c81428c 606
19799a22 607If the first character after the B<-M> or B<-m> is a dash (C<->)
a5f75d66 608then the 'use' is replaced with 'no'.
609
54310121 610A little builtin syntactic sugar means you can also say
19799a22 611B<-mmodule=foo,bar> or B<-Mmodule=foo,bar> as a shortcut for
612C<'-Mmodule qw(foo bar)'>. This avoids the need to use quotes when
613importing symbols. The actual code generated by B<-Mmodule=foo,bar> is
e0ebc809 614C<use module split(/,/,q{foo,bar})>. Note that the C<=> form
19799a22 615removes the distinction between B<-m> and B<-M>.
3c81428c 616
a0d0e21e 617=item B<-n>
618
19799a22 619causes Perl to assume the following loop around your program, which
a0d0e21e 620makes it iterate over filename arguments somewhat like B<sed -n> or
621B<awk>:
622
19799a22 623 LINE:
a0d0e21e 624 while (<>) {
19799a22 625 ... # your program goes here
a0d0e21e 626 }
627
628Note that the lines are not printed by default. See B<-p> to have
08e9d68e 629lines printed. If a file named by an argument cannot be opened for
19799a22 630some reason, Perl warns you about it and moves on to the next file.
08e9d68e 631
fa11829f 632Here is an efficient way to delete all files that haven't been modified for
9976c5c7 633at least a week:
a0d0e21e 634
19799a22 635 find . -mtime +7 -print | perl -nle unlink
a0d0e21e 636
19799a22 637This is faster than using the B<-exec> switch of B<find> because you don't
638have to start a process on every filename found. It does suffer from
639the bug of mishandling newlines in pathnames, which you can fix if
44a4342c 640you follow the example under B<-0>.
a0d0e21e 641
642C<BEGIN> and C<END> blocks may be used to capture control before or after
19799a22 643the implicit program loop, just as in B<awk>.
a0d0e21e 644
645=item B<-p>
646
19799a22 647causes Perl to assume the following loop around your program, which
a0d0e21e 648makes it iterate over filename arguments somewhat like B<sed>:
649
650
19799a22 651 LINE:
a0d0e21e 652 while (<>) {
19799a22 653 ... # your program goes here
a0d0e21e 654 } continue {
08e9d68e 655 print or die "-p destination: $!\n";
a0d0e21e 656 }
657
08e9d68e 658If a file named by an argument cannot be opened for some reason, Perl
659warns you about it, and moves on to the next file. Note that the
c2611fb3 660lines are printed automatically. An error occurring during printing is
08e9d68e 661treated as fatal. To suppress printing use the B<-n> switch. A B<-p>
662overrides a B<-n> switch.
a0d0e21e 663
664C<BEGIN> and C<END> blocks may be used to capture control before or after
19799a22 665the implicit loop, just as in B<awk>.
a0d0e21e 666
667=item B<-P>
668
079a94c4 669B<NOTE: Use of -P is strongly discouraged because of its inherent
670problems, including poor portability.>
671
672This option causes your program to be run through the C preprocessor before
efdf3af0 673compilation by Perl. Because both comments and B<cpp> directives begin
a0d0e21e 674with the # character, you should avoid starting comments with any words
efdf3af0 675recognized by the C preprocessor such as C<"if">, C<"else">, or C<"define">.
079a94c4 676
677If you're considering using C<-P>, you might also want to look at the
678Filter::cpp module from CPAN.
679
680The problems of -P include, but are not limited to:
681
682=over 10
683
684=item *
685
686The C<#!> line is stripped, so any switches there don't apply.
687
688=item *
689
690A C<-P> on a C<#!> line doesn't work.
691
692=item *
693
694B<All> lines that begin with (whitespace and) a C<#> but
695do not look like cpp commands, are stripped, including anything
44a4342c 696inside Perl strings, regular expressions, and here-docs .
079a94c4 697
698=item *
699
700In some platforms the C preprocessor knows too much: it knows about
701the C++ -style until-end-of-line comments starting with C<"//">.
efdf3af0 702This will cause problems with common Perl constructs like
703
704 s/foo//;
705
706because after -P this will became illegal code
707
708 s/foo
709
710The workaround is to use some other quoting separator than C<"/">,
711like for example C<"!">:
712
713 s!foo!!;
a0d0e21e 714
079a94c4 715
716
717=item *
718
719It requires not only a working C preprocessor but also a working
720F<sed>. If not on UNIX, you are probably out of luck on this.
721
722=item *
723
724Script line numbers are not preserved.
725
726=item *
727
728The C<-x> does not work with C<-P>.
729
730=back
9a1f07e7 731
a0d0e21e 732=item B<-s>
733
19799a22 734enables rudimentary switch parsing for switches on the command
735line after the program name but before any filename arguments (or before
3bbcc830 736an argument of B<-->). This means you can have switches with two leading
737dashes (B<--help>). Any switch found there is removed from @ARGV and sets the
19799a22 738corresponding variable in the Perl program. The following program
3c0facb2 739prints "1" if the program is invoked with a B<-xyz> switch, and "abc"
740if it is invoked with B<-xyz=abc>.
a0d0e21e 741
742 #!/usr/bin/perl -s
3c0facb2 743 if ($xyz) { print "$xyz\n" }
a0d0e21e 744
3bbcc830 745Do note that B<--help> creates the variable ${-help}, which is not compliant
50b5b186 746with C<strict refs>. Also, when using this option on a script with
747warnings enabled you may get a lot of spurious "used only once" warnings.
3bbcc830 748
a0d0e21e 749=item B<-S>
750
751makes Perl use the PATH environment variable to search for the
19799a22 752program (unless the name of the program contains directory separators).
753
2a92aaa0 754On some platforms, this also makes Perl append suffixes to the
755filename while searching for it. For example, on Win32 platforms,
756the ".bat" and ".cmd" suffixes are appended if a lookup for the
757original name fails, and if the name does not already end in one
758of those suffixes. If your Perl was compiled with DEBUGGING turned
759on, using the -Dp switch to Perl shows how the search progresses.
760
fa3aa65a 761Typically this is used to emulate #! startup on platforms that don't
762support #!. Its also convenient when debugging a script that uses #!,
763and is thus normally found by the shell's $PATH search mechanism.
764
765This example works on many platforms that have a shell compatible with
766Bourne shell:
a0d0e21e 767
768 #!/usr/bin/perl
a3cb178b 769 eval 'exec /usr/bin/perl -wS $0 ${1+"$@"}'
a0d0e21e 770 if $running_under_some_shell;
771
19799a22 772The system ignores the first line and feeds the program to F</bin/sh>,
773which proceeds to try to execute the Perl program as a shell script.
a0d0e21e 774The shell executes the second line as a normal shell command, and thus
775starts up the Perl interpreter. On some systems $0 doesn't always
776contain the full pathname, so the B<-S> tells Perl to search for the
19799a22 777program if necessary. After Perl locates the program, it parses the
a0d0e21e 778lines and ignores them because the variable $running_under_some_shell
19799a22 779is never true. If the program will be interpreted by csh, you will need
a3cb178b 780to replace C<${1+"$@"}> with C<$*>, even though that doesn't understand
781embedded spaces (and such) in the argument list. To start up sh rather
a0d0e21e 782than csh, some systems may have to replace the #! line with a line
783containing just a colon, which will be politely ignored by Perl. Other
784systems can't control that, and need a totally devious construct that
19799a22 785will work under any of B<csh>, B<sh>, or Perl, such as the following:
a0d0e21e 786
19799a22 787 eval '(exit $?0)' && eval 'exec perl -wS $0 ${1+"$@"}'
a3cb178b 788 & eval 'exec /usr/bin/perl -wS $0 $argv:q'
5f05dabc 789 if $running_under_some_shell;
a0d0e21e 790
19799a22 791If the filename supplied contains directory separators (i.e., is an
792absolute or relative pathname), and if that file is not found,
793platforms that append file extensions will do so and try to look
794for the file with those extensions added, one by one.
795
796On DOS-like platforms, if the program does not contain directory
797separators, it will first be searched for in the current directory
798before being searched for on the PATH. On Unix platforms, the
799program will be searched for strictly on the PATH.
800
6537fe72 801=item B<-t>
802
803Like B<-T>, but taint checks will issue warnings rather than fatal
317ea90d 804errors. These warnings can be controlled normally with C<no warnings
805qw(taint)>.
1dbad523 806
807B<NOTE: this is not a substitute for -T.> This is meant only to be
808used as a temporary development aid while securing legacy code:
809for real production code and for new secure code written from scratch
810always use the real B<-T>.
6537fe72 811
a0d0e21e 812=item B<-T>
813
a3cb178b 814forces "taint" checks to be turned on so you can test them. Ordinarily
19799a22 815these checks are done only when running setuid or setgid. It's a
816good idea to turn them on explicitly for programs that run on behalf
817of someone else whom you might not necessarily trust, such as CGI
818programs or any internet servers you might write in Perl. See
819L<perlsec> for details. For security reasons, this option must be
820seen by Perl quite early; usually this means it must appear early
821on the command line or in the #! line for systems which support
822that construct.
a0d0e21e 823
824=item B<-u>
825
19799a22 826This obsolete switch causes Perl to dump core after compiling your
827program. You can then in theory take this core dump and turn it
828into an executable file by using the B<undump> program (not supplied).
829This speeds startup at the expense of some disk space (which you
830can minimize by stripping the executable). (Still, a "hello world"
831executable comes out to about 200K on my machine.) If you want to
832execute a portion of your program before dumping, use the dump()
833operator instead. Note: availability of B<undump> is platform
834specific and may not be available for a specific port of Perl.
835
836This switch has been superseded in favor of the new Perl code
837generator backends to the compiler. See L<B> and L<B::Bytecode>
838for details.
a0d0e21e 839
840=item B<-U>
841
842allows Perl to do unsafe operations. Currently the only "unsafe"
843operations are the unlinking of directories while running as superuser,
844and running setuid programs with fatal taint checks turned into
19799a22 845warnings. Note that the B<-w> switch (or the C<$^W> variable) must
846be used along with this option to actually I<generate> the
fb73857a 847taint-check warnings.
a0d0e21e 848
849=item B<-v>
850
19799a22 851prints the version and patchlevel of your perl executable.
a0d0e21e 852
3c81428c 853=item B<-V>
854
855prints summary of the major perl configuration values and the current
19799a22 856values of @INC.
3c81428c 857
307dc113 858=item B<-V:>I<configvar>
3c81428c 859
4a305f6a 860Prints to STDOUT the value of the named configuration variable(s),
307dc113 861with multiples when your configvar argument looks like a regex (has
862non-letters). For example:
3c81428c 863
307dc113 864 $ perl -V:libc
865 libc='/lib/libc-2.2.4.so';
4a305f6a 866 $ perl -V:lib.
867 libs='-lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc';
868 libc='/lib/libc-2.2.4.so';
869 $ perl -V:lib.*
870 libpth='/usr/local/lib /lib /usr/lib';
871 libs='-lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc';
872 lib_ext='.a';
873 libc='/lib/libc-2.2.4.so';
874 libperl='libperl.a';
875 ....
876
877Additionally, extra colons can be used to control formatting. A
878trailing colon suppresses the linefeed and terminator ';', allowing
879you to embed queries into shell commands. (mnemonic: PATH separator
880':'.)
881
882 $ echo "compression-vars: " `perl -V:z.*: ` " are here !"
883 compression-vars: zcat='' zip='zip' are here !
884
885A leading colon removes the 'name=' part of the response, this allows
307dc113 886you to map to the name you need. (mnemonic: empty label)
4a305f6a 887
888 $ echo "goodvfork="`./perl -Ilib -V::usevfork`
889 goodvfork=false;
890
891Leading and trailing colons can be used together if you need
892positional parameter values without the names. Note that in the case
893below, the PERL_API params are returned in alphabetical order.
894
895 $ echo building_on `perl -V::osname: -V::PERL_API_.*:` now
896 building_on 'linux' '5' '1' '9' now
a0d0e21e 897
19799a22 898=item B<-w>
774d564b 899
19799a22 900prints warnings about dubious constructs, such as variable names
901that are mentioned only once and scalar variables that are used
902before being set, redefined subroutines, references to undefined
903filehandles or filehandles opened read-only that you are attempting
904to write on, values used as a number that doesn't look like numbers,
905using an array as though it were a scalar, if your subroutines
906recurse more than 100 deep, and innumerable other things.
907
b40da996 908This switch really just enables the internal C<$^W> variable. You
19799a22 909can disable or promote into fatal errors specific warnings using
910C<__WARN__> hooks, as described in L<perlvar> and L<perlfunc/warn>.
911See also L<perldiag> and L<perltrap>. A new, fine-grained warning
912facility is also available if you want to manipulate entire classes
9f1b1f2d 913of warnings; see L<warnings> or L<perllexwarn>.
a0d0e21e 914
0453d815 915=item B<-W>
916
3c0facb2 917Enables all warnings regardless of C<no warnings> or C<$^W>.
0453d815 918See L<perllexwarn>.
919
920=item B<-X>
921
3c0facb2 922Disables all warnings regardless of C<use warnings> or C<$^W>.
0453d815 923See L<perllexwarn>.
924
136e4fd6 925=item B<-x>
926
a0d0e21e 927=item B<-x> I<directory>
928
19799a22 929tells Perl that the program is embedded in a larger chunk of unrelated
930ASCII text, such as in a mail message. Leading garbage will be
931discarded until the first line that starts with #! and contains the
932string "perl". Any meaningful switches on that line will be applied.
933If a directory name is specified, Perl will switch to that directory
934before running the program. The B<-x> switch controls only the
935disposal of leading garbage. The program must be terminated with
936C<__END__> if there is trailing garbage to be ignored (the program
937can process any or all of the trailing garbage via the DATA filehandle
938if desired).
a0d0e21e 939
1e422769 940=back
941
942=head1 ENVIRONMENT
943
944=over 12
945
946=item HOME
947
948Used if chdir has no argument.
949
950=item LOGDIR
951
952Used if chdir has no argument and HOME is not set.
953
954=item PATH
955
19799a22 956Used in executing subprocesses, and in finding the program if B<-S> is
1e422769 957used.
958
959=item PERL5LIB
960
48b971ca 961A list of directories in which to look for Perl library
1e422769 962files before looking in the standard library and the current
951ba7fe 963directory. Any architecture-specific directories under the specified
964locations are automatically included if they exist. If PERL5LIB is not
48b971ca 965defined, PERLLIB is used. Directories are separated (like in PATH) by
966a colon on unixish platforms and by a semicolon on Windows (the proper
967path separator being given by the command C<perl -V:path_sep>).
951ba7fe 968
969When running taint checks (either because the program was running setuid
970or setgid, or the B<-T> switch was used), neither variable is used.
971The program should instead say:
1e422769 972
973 use lib "/my/directory";
974
54310121 975=item PERL5OPT
976
977Command-line options (switches). Switches in this variable are taken
1c4db469 978as if they were on every Perl command line. Only the B<-[DIMUdmtw]>
19799a22 979switches are allowed. When running taint checks (because the program
54310121 980was running setuid or setgid, or the B<-T> switch was used), this
74288ac8 981variable is ignored. If PERL5OPT begins with B<-T>, tainting will be
982enabled, and any subsequent options ignored.
54310121 983
16537909 984=item PERLIO
985
44a4342c 986A space (or colon) separated list of PerlIO layers. If perl is built
03d9e98a 987to use PerlIO system for IO (the default) these layers effect perl's IO.
44a4342c 988
989It is conventional to start layer names with a colon e.g. C<:perlio> to
990emphasise their similarity to variable "attributes". But the code that parses
991layer specification strings (which is also used to decode the PERLIO
992environment variable) treats the colon as a separator.
993
3b0db4f9 994An unset or empty PERLIO is equivalent to C<:stdio>.
995
44a4342c 996The list becomes the default for I<all> perl's IO. Consequently only built-in
997layers can appear in this list, as external layers (such as :encoding()) need
998IO in order to load them!. See L<"open pragma"|open> for how to add external
999encodings as defaults.
1000
1001The layers that it makes sense to include in the PERLIO environment
3d897973 1002variable are briefly summarised below. For more details see L<PerlIO>.
16537909 1003
1004=over 8
1005
1006=item :bytes
1007
18aba96f 1008A pseudolayer that turns I<off> the C<:utf8> flag for the layer below.
1009Unlikely to be useful on its own in the global PERLIO environment variable.
1010You perhaps were thinking of C<:crlf:bytes> or C<:perlio:bytes>.
16537909 1011
1012=item :crlf
1013
3d897973 1014A layer which does CRLF to "\n" translation distinguishing "text" and
1015"binary" files in the manner of MS-DOS and similar operating systems.
1016(It currently does I<not> mimic MS-DOS as far as treating of Control-Z
1017as being an end-of-file marker.)
44a4342c 1018
1019=item :mmap
1020
1021A layer which implements "reading" of files by using C<mmap()> to
1022make (whole) file appear in the process's address space, and then
3d897973 1023using that as PerlIO's "buffer".
16537909 1024
44a4342c 1025=item :perlio
16537909 1026
3d897973 1027This is a re-implementation of "stdio-like" buffering written as a
1028PerlIO "layer". As such it will call whatever layer is below it for
1029its operations (typically C<:unix>).
16537909 1030
18aba96f 1031=item :pop
1032
1033An experimental pseudolayer that removes the topmost layer.
3d897973 1034Use with the same care as is reserved for nitroglycerin.
18aba96f 1035
44a4342c 1036=item :raw
16537909 1037
136e4fd6 1038A pseudolayer that manipulates other layers. Applying the C<:raw>
18aba96f 1039layer is equivalent to calling C<binmode($fh)>. It makes the stream
1040pass each byte as-is without any translation. In particular CRLF
1041translation, and/or :utf8 intuited from locale are disabled.
1cbfc93d 1042
3d897973 1043Unlike in the earlier versions of Perl C<:raw> is I<not>
1044just the inverse of C<:crlf> - other layers which would affect the
1045binary nature of the stream are also removed or disabled.
16537909 1046
44a4342c 1047=item :stdio
1048
1049This layer provides PerlIO interface by wrapping system's ANSI C "stdio"
1050library calls. The layer provides both buffering and IO.
1051Note that C<:stdio> layer does I<not> do CRLF translation even if that
1052is platforms normal behaviour. You will need a C<:crlf> layer above it
1053to do that.
1054
1055=item :unix
1056
3d897973 1057Low level layer which calls C<read>, C<write> and C<lseek> etc.
16537909 1058
1059=item :utf8
1060
18aba96f 1061A pseudolayer that turns on a flag on the layer below to tell perl
3d897973 1062that output should be in utf8 and that input should be regarded as
1063already in utf8 form. May be useful in PERLIO environment
1064variable to make UTF-8 the default. (To turn off that behaviour
1065use C<:bytes> layer.)
44a4342c 1066
1067=item :win32
1068
ab4f7683 1069On Win32 platforms this I<experimental> layer uses native "handle" IO
44a4342c 1070rather than unix-like numeric file descriptor layer. Known to be
1071buggy in this release.
16537909 1072
1073=back
1074
44a4342c 1075On all platforms the default set of layers should give acceptable results.
1076
ab4f7683 1077For UNIX platforms that will equivalent of "unix perlio" or "stdio".
44a4342c 1078Configure is setup to prefer "stdio" implementation if system's library
1079provides for fast access to the buffer, otherwise it uses the "unix perlio"
1080implementation.
1081
1082On Win32 the default in this release is "unix crlf". Win32's "stdio"
1083has a number of bugs/mis-features for perl IO which are somewhat
99366417 1084C compiler vendor/version dependent. Using our own C<crlf> layer as
44a4342c 1085the buffer avoids those issues and makes things more uniform.
1086The C<crlf> layer provides CRLF to/from "\n" conversion as well as
1087buffering.
1088
1089This release uses C<unix> as the bottom layer on Win32 and so still uses C
1090compiler's numeric file descriptor routines. There is an experimental native
3d897973 1091C<win32> layer which is expected to be enhanced and should eventually be
1092the default under Win32.
44a4342c 1093
1094=item PERLIO_DEBUG
1095
1096If set to the name of a file or device then certain operations of PerlIO
1097sub-system will be logged to that file (opened as append). Typical uses
1098are UNIX:
1099
1100 PERLIO_DEBUG=/dev/tty perl script ...
1101
1102and Win32 approximate equivalent:
1103
1104 set PERLIO_DEBUG=CON
1105 perl script ...
1106
923e8b21 1107This functionality is disabled for setuid scripts and for scripts run
1108with B<-T>.
16537909 1109
1e422769 1110=item PERLLIB
1111
48b971ca 1112A list of directories in which to look for Perl library
1e422769 1113files before looking in the standard library and the current directory.
1114If PERL5LIB is defined, PERLLIB is not used.
1115
1116=item PERL5DB
1117
1118The command used to load the debugger code. The default is:
1119
1120 BEGIN { require 'perl5db.pl' }
1121
2cbb2ee1 1122=item PERL5DB_THREADED
1123
1124If set to a true value, indicates to the debugger that the code being
1125debugged uses threads.
1126
19799a22 1127=item PERL5SHELL (specific to the Win32 port)
174c211a 1128
1129May be set to an alternative shell that perl must use internally for
11998fdb 1130executing "backtick" commands or system(). Default is C<cmd.exe /x/d/c>
ce1da67e 1131on WindowsNT and C<command.com /c> on Windows95. The value is considered
19799a22 1132to be space-separated. Precede any character that needs to be protected
ce1da67e 1133(like a space or backslash) with a backslash.
1134
1135Note that Perl doesn't use COMSPEC for this purpose because
1136COMSPEC has a high degree of variability among users, leading to
1137portability concerns. Besides, perl can use a shell that may not be
1138fit for interactive use, and setting COMSPEC to such a shell may
1139interfere with the proper functioning of other programs (which usually
1140look in COMSPEC to find a shell fit for interactive use).
174c211a 1141
1c972609 1142=item PERL_ALLOW_NON_IFS_LSP (specific to the Win32 port)
1143
1144Set to 1 to allow the use of non-IFS compatible LSP's.
1145Perl normally searches for an IFS-compatible LSP because this is required
1146for its emulation of Windows sockets as real filehandles. However, this may
1147cause problems if you have a firewall such as McAfee Guardian which requires
1148all applications to use its LSP which is not IFS-compatible, because clearly
1149Perl will normally avoid using such an LSP.
1150Setting this environment variable to 1 means that Perl will simply use the
1151first suitable LSP enumerated in the catalog, which keeps McAfee Guardian
1152happy (and in that particular case Perl still works too because McAfee
1153Guardian's LSP actually plays some other games which allow applications
1154requiring IFS compatibility to work).
1155
1e422769 1156=item PERL_DEBUG_MSTATS
1157
67ce8856 1158Relevant only if perl is compiled with the malloc included with the perl
a3cb178b 1159distribution (that is, if C<perl -V:d_mymalloc> is 'define').
1160If set, this causes memory statistics to be dumped after execution. If set
1e422769 1161to an integer greater than one, also causes memory statistics to be dumped
1162after compilation.
1163
1164=item PERL_DESTRUCT_LEVEL
1165
1166Relevant only if your perl executable was built with B<-DDEBUGGING>,
1167this controls the behavior of global destruction of objects and other
64cea5fd 1168references. See L<perlhack/PERL_DESTRUCT_LEVEL> for more information.
a0d0e21e 1169
02c7413a 1170=item PERL_DL_NONLAZY
1171
1172Set to one to have perl resolve B<all> undefined symbols when it loads
1173a dynamic library. The default behaviour is to resolve symbols when
1174they are used. Setting this variable is useful during testing of
1175extensions as it ensures that you get an error on misspelled function
1176names even if the test suite doesn't call it.
1177
5d170f3a 1178=item PERL_ENCODING
1179
1180If using the C<encoding> pragma without an explicit encoding name, the
1181PERL_ENCODING environment variable is consulted for an encoding name.
1182
504f80c1 1183=item PERL_HASH_SEED
1184
183c3da1 1185(Since Perl 5.8.1.) Used to randomise Perl's internal hash function.
4546b9e6 1186To emulate the pre-5.8.1 behaviour, set to an integer (zero means
1187exactly the same order as 5.8.0). "Pre-5.8.1" means, among other
1188things, that hash keys will be ordered the same between different runs
1189of Perl.
504f80c1 1190
4546b9e6 1191The default behaviour is to randomise unless the PERL_HASH_SEED is set.
1192If Perl has been compiled with C<-DUSE_HASH_SEED_EXPLICIT>, the default
1193behaviour is B<not> to randomise unless the PERL_HASH_SEED is set.
504f80c1 1194
1195If PERL_HASH_SEED is unset or set to a non-numeric string, Perl uses
1196the pseudorandom seed supplied by the operating system and libraries.
4546b9e6 1197This means that each different run of Perl will have a different
1198ordering of the results of keys(), values(), and each().
504f80c1 1199
26a2d347 1200B<Please note that the hash seed is sensitive information>. Hashes are
1201randomized to protect against local and remote attacks against Perl
1202code. By manually setting a seed this protection may be partially or
1203completely lost.
1204
1205See L<perlsec/"Algorithmic Complexity Attacks"> and
1206L</PERL_HASH_SEED_DEBUG> for more information.
504f80c1 1207
2191697e 1208=item PERL_HASH_SEED_DEBUG
1209
e67b9e52 1210(Since Perl 5.8.1.) Set to one to display (to STDERR) the value of
26a2d347 1211the hash seed at the beginning of execution. This, combined with
1212L</PERL_HASH_SEED> is intended to aid in debugging nondeterministic
1213behavior caused by hash randomization.
1214
1215B<Note that the hash seed is sensitive information>: by knowing it one
1216can craft a denial-of-service attack against Perl code, even remotely,
1217see L<perlsec/"Algorithmic Complexity Attacks"> for more information.
e67b9e52 1218B<Do not disclose the hash seed> to people who don't need to know it.
9a7034eb 1219See also hash_seed() of L<Hash::Util>.
2191697e 1220
3d0ae7ba 1221=item PERL_ROOT (specific to the VMS port)
1222
1223A translation concealed rooted logical name that contains perl and the
1224logical device for the @INC path on VMS only. Other logical names that
44a4342c 1225affect perl on VMS include PERLSHR, PERL_ENV_TABLES, and
1226SYS$TIMEZONE_DIFFERENTIAL but are optional and discussed further in
3d0ae7ba 1227L<perlvms> and in F<README.vms> in the Perl source distribution.
1228
4ffa73a3 1229=item PERL_SIGNALS
1230
1231In Perls 5.8.1 and later. If set to C<unsafe> the pre-Perl-5.8.0
1232signals behaviour (immediate but unsafe) is restored. If set to
ec488bcf 1233C<safe> the safe (or deferred) signals are used.
65c3f8ef 1234See L<perlipc/"Deferred Signals (Safe Signals)">.
4ffa73a3 1235
a05d7ebb 1236=item PERL_UNICODE
acae81db 1237
bf61ac64 1238Equivalent to the B<-C> command-line switch. Note that this is not
1239a boolean variable-- setting this to C<"1"> is not the right way to
5b4f334e 1240"enable Unicode" (whatever that would mean). You can use C<"0"> to
e654d908 1241"disable Unicode", though (or alternatively unset PERL_UNICODE in
1242your shell before starting Perl). See the description of the C<-C>
1243switch for more information.
acae81db 1244
3d0ae7ba 1245=item SYS$LOGIN (specific to the VMS port)
1246
1247Used if chdir has no argument and HOME and LOGDIR are not set.
1248
a0d0e21e 1249=back
1e422769 1250
1251Perl also has environment variables that control how Perl handles data
1252specific to particular natural languages. See L<perllocale>.
1253
1254Apart from these, Perl uses no other environment variables, except
19799a22 1255to make them available to the program being executed, and to child
1256processes. However, programs running setuid would do well to execute
1e422769 1257the following lines before doing anything else, just to keep people
1258honest:
1259
19799a22 1260 $ENV{PATH} = '/bin:/usr/bin'; # or whatever you need
7bac28a0 1261 $ENV{SHELL} = '/bin/sh' if exists $ENV{SHELL};
c90c0ff4 1262 delete @ENV{qw(IFS CDPATH ENV BASH_ENV)};