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1=head1 NAME
2
3perlsyn - Perl syntax
4
5=head1 DESCRIPTION
6
7A Perl script consists of a sequence of declarations and statements.
8The only things that need to be declared in Perl are report formats
9and subroutines. See the sections below for more information on those
10declarations. All uninitialized user-created objects are assumed to
11start with a null or 0 value until they are defined by some explicit
12operation such as assignment. (Though you can get warnings about the
13use of undefined values if you like.) The sequence of statements is
14executed just once, unlike in B<sed> and B<awk> scripts, where the
15sequence of statements is executed for each input line. While this means
16that you must explicitly loop over the lines of your input file (or
17files), it also means you have much more control over which files and
18which lines you look at. (Actually, I'm lying--it is possible to do an
19implicit loop with either the B<-n> or B<-p> switch. It's just not the
20mandatory default like it is in B<sed> and B<awk>.)
21
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22=head2 Declarations
23
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24Perl is, for the most part, a free-form language. (The only
25exception to this is format declarations, for obvious reasons.) Comments
26are indicated by the "#" character, and extend to the end of the line. If
27you attempt to use C</* */> C-style comments, it will be interpreted
28either as division or pattern matching, depending on the context, and C++
4633a7c4 29C<//> comments just look like a null regular expression, so don't do
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30that.
31
32A declaration can be put anywhere a statement can, but has no effect on
33the execution of the primary sequence of statements--declarations all
34take effect at compile time. Typically all the declarations are put at
54310121 35the beginning or the end of the script. However, if you're using
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36lexically-scoped private variables created with my(), you'll have to make sure
37your format or subroutine definition is within the same block scope
5f05dabc 38as the my if you expect to be able to access those private variables.
a0d0e21e 39
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40Declaring a subroutine allows a subroutine name to be used as if it were a
41list operator from that point forward in the program. You can declare a
54310121 42subroutine without defining it by saying C<sub name>, thus:
a0d0e21e 43
54310121 44 sub myname;
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45 $me = myname $0 or die "can't get myname";
46
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47Note that it functions as a list operator, not as a unary operator; so
48be careful to use C<or> instead of C<||> in this case. However, if
49you were to declare the subroutine as C<sub myname ($)>, then
50C<myname> would functonion as a unary operator, so either C<or> or
51C<||> would work.
a0d0e21e 52
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53Subroutines declarations can also be loaded up with the C<require> statement
54or both loaded and imported into your namespace with a C<use> statement.
55See L<perlmod> for details on this.
a0d0e21e 56
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57A statement sequence may contain declarations of lexically-scoped
58variables, but apart from declaring a variable name, the declaration acts
59like an ordinary statement, and is elaborated within the sequence of
60statements as if it were an ordinary statement. That means it actually
61has both compile-time and run-time effects.
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62
63=head2 Simple statements
64
65The only kind of simple statement is an expression evaluated for its
66side effects. Every simple statement must be terminated with a
67semicolon, unless it is the final statement in a block, in which case
68the semicolon is optional. (A semicolon is still encouraged there if the
5f05dabc 69block takes up more than one line, because you may eventually add another line.)
a0d0e21e 70Note that there are some operators like C<eval {}> and C<do {}> that look
54310121 71like compound statements, but aren't (they're just TERMs in an expression),
4633a7c4 72and thus need an explicit termination if used as the last item in a statement.
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73
74Any simple statement may optionally be followed by a I<SINGLE> modifier,
75just before the terminating semicolon (or block ending). The possible
76modifiers are:
77
78 if EXPR
79 unless EXPR
80 while EXPR
81 until EXPR
82
83The C<if> and C<unless> modifiers have the expected semantics,
84presuming you're a speaker of English. The C<while> and C<until>
85modifiers also have the usual "while loop" semantics (conditional
86evaluated first), except when applied to a do-BLOCK (or to the
87now-deprecated do-SUBROUTINE statement), in which case the block
88executes once before the conditional is evaluated. This is so that you
89can write loops like:
90
91 do {
4633a7c4 92 $line = <STDIN>;
a0d0e21e 93 ...
4633a7c4 94 } until $line eq ".\n";
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95
96See L<perlfunc/do>. Note also that the loop control
5f05dabc 97statements described later will I<NOT> work in this construct, because
a0d0e21e 98modifiers don't take loop labels. Sorry. You can always wrap
4633a7c4 99another block around it to do that sort of thing.
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100
101=head2 Compound statements
102
103In Perl, a sequence of statements that defines a scope is called a block.
104Sometimes a block is delimited by the file containing it (in the case
105of a required file, or the program as a whole), and sometimes a block
106is delimited by the extent of a string (in the case of an eval).
107
108But generally, a block is delimited by curly brackets, also known as braces.
109We will call this syntactic construct a BLOCK.
110
111The following compound statements may be used to control flow:
112
113 if (EXPR) BLOCK
114 if (EXPR) BLOCK else BLOCK
115 if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
116 LABEL while (EXPR) BLOCK
117 LABEL while (EXPR) BLOCK continue BLOCK
118 LABEL for (EXPR; EXPR; EXPR) BLOCK
748a9306 119 LABEL foreach VAR (LIST) BLOCK
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120 LABEL BLOCK continue BLOCK
121
122Note that, unlike C and Pascal, these are defined in terms of BLOCKs,
123not statements. This means that the curly brackets are I<required>--no
124dangling statements allowed. If you want to write conditionals without
125curly brackets there are several other ways to do it. The following
126all do the same thing:
127
128 if (!open(FOO)) { die "Can't open $FOO: $!"; }
129 die "Can't open $FOO: $!" unless open(FOO);
130 open(FOO) or die "Can't open $FOO: $!"; # FOO or bust!
131 open(FOO) ? 'hi mom' : die "Can't open $FOO: $!";
132 # a bit exotic, that last one
133
5f05dabc 134The C<if> statement is straightforward. Because BLOCKs are always
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135bounded by curly brackets, there is never any ambiguity about which
136C<if> an C<else> goes with. If you use C<unless> in place of C<if>,
137the sense of the test is reversed.
138
139The C<while> statement executes the block as long as the expression is
140true (does not evaluate to the null string or 0 or "0"). The LABEL is
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141optional, and if present, consists of an identifier followed by a colon.
142The LABEL identifies the loop for the loop control statements C<next>,
143C<last>, and C<redo>. If the LABEL is omitted, the loop control statement
144refers to the innermost enclosing loop. This may include dynamically
145looking back your call-stack at run time to find the LABEL. Such
146desperate behavior triggers a warning if you use the B<-w> flag.
147
148If there is a C<continue> BLOCK, it is always executed just before the
149conditional is about to be evaluated again, just like the third part of a
150C<for> loop in C. Thus it can be used to increment a loop variable, even
151when the loop has been continued via the C<next> statement (which is
152similar to the C C<continue> statement).
153
154=head2 Loop Control
155
156The C<next> command is like the C<continue> statement in C; it starts
157the next iteration of the loop:
158
159 LINE: while (<STDIN>) {
160 next LINE if /^#/; # discard comments
161 ...
162 }
163
164The C<last> command is like the C<break> statement in C (as used in
165loops); it immediately exits the loop in question. The
166C<continue> block, if any, is not executed:
167
168 LINE: while (<STDIN>) {
169 last LINE if /^$/; # exit when done with header
170 ...
171 }
172
173The C<redo> command restarts the loop block without evaluating the
174conditional again. The C<continue> block, if any, is I<not> executed.
175This command is normally used by programs that want to lie to themselves
176about what was just input.
177
178For example, when processing a file like F</etc/termcap>.
179If your input lines might end in backslashes to indicate continuation, you
180want to skip ahead and get the next record.
181
182 while (<>) {
183 chomp;
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184 if (s/\\$//) {
185 $_ .= <>;
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186 redo unless eof();
187 }
188 # now process $_
54310121 189 }
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190
191which is Perl short-hand for the more explicitly written version:
192
54310121 193 LINE: while (defined($line = <ARGV>)) {
4633a7c4 194 chomp($line);
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195 if ($line =~ s/\\$//) {
196 $line .= <ARGV>;
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197 redo LINE unless eof(); # not eof(ARGV)!
198 }
199 # now process $line
54310121 200 }
4633a7c4 201
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202Or here's a simpleminded Pascal comment stripper (warning: assumes no
203{ or } in strings).
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204
205 LINE: while (<STDIN>) {
206 while (s|({.*}.*){.*}|$1 |) {}
207 s|{.*}| |;
208 if (s|{.*| |) {
209 $front = $_;
210 while (<STDIN>) {
211 if (/}/) { # end of comment?
212 s|^|$front{|;
213 redo LINE;
214 }
215 }
216 }
217 print;
218 }
219
220Note that if there were a C<continue> block on the above code, it would get
221executed even on discarded lines.
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222
223If the word C<while> is replaced by the word C<until>, the sense of the
224test is reversed, but the conditional is still tested before the first
225iteration.
226
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227The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer
228available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>.
4633a7c4 229
cb1a09d0 230=head2 For Loops
a0d0e21e 231
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232Perl's C-style C<for> loop works exactly like the corresponding C<while> loop;
233that means that this:
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234
235 for ($i = 1; $i < 10; $i++) {
236 ...
237 }
238
cb1a09d0 239is the same as this:
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240
241 $i = 1;
242 while ($i < 10) {
243 ...
244 } continue {
245 $i++;
246 }
247
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248(There is one minor difference: The first form implies a lexical scope
249for variables declared with C<my> in the initialization expression.)
250
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251Besides the normal array index looping, C<for> can lend itself
252to many other interesting applications. Here's one that avoids the
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253problem you get into if you explicitly test for end-of-file on
254an interactive file descriptor causing your program to appear to
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255hang.
256
257 $on_a_tty = -t STDIN && -t STDOUT;
258 sub prompt { print "yes? " if $on_a_tty }
259 for ( prompt(); <STDIN>; prompt() ) {
260 # do something
54310121 261 }
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262
263=head2 Foreach Loops
264
4633a7c4 265The C<foreach> loop iterates over a normal list value and sets the
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266variable VAR to be each element of the list in turn. If the variable
267is preceded with the keyword C<my>, then it is lexically scoped, and
268is therefore visible only within the loop. Otherwise, the variable is
269implicitly local to the loop and regains its former value upon exiting
270the loop. If the variable was previously declared with C<my>, it uses
271that variable instead of the global one, but it's still localized to
272the loop. (Note that a lexically scoped variable can cause problems
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273if you have subroutine or format declarations within the loop which
274refer to it.)
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275
276The C<foreach> keyword is actually a synonym for the C<for> keyword, so
277you can use C<foreach> for readability or C<for> for brevity. If VAR is
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278omitted, $_ is set to each value. If any element of LIST is an lvalue,
279you can modify it by modifying VAR inside the loop. That's because
280the C<foreach> loop index variable is an implicit alias for each item
281in the list that you're looping over.
282
283If any part of LIST is an array, C<foreach> will get very confused if
284you add or remove elements within the loop body, for example with
285C<splice>. So don't do that.
286
287C<foreach> probably won't do what you expect if VAR is a tied or other
288special variable. Don't do that either.
4633a7c4 289
748a9306 290Examples:
a0d0e21e 291
4633a7c4 292 for (@ary) { s/foo/bar/ }
a0d0e21e 293
55497cff 294 foreach my $elem (@elements) {
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295 $elem *= 2;
296 }
297
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298 for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') {
299 print $count, "\n"; sleep(1);
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300 }
301
302 for (1..15) { print "Merry Christmas\n"; }
303
4633a7c4 304 foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) {
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305 print "Item: $item\n";
306 }
307
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308Here's how a C programmer might code up a particular algorithm in Perl:
309
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310 for (my $i = 0; $i < @ary1; $i++) {
311 for (my $j = 0; $j < @ary2; $j++) {
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312 if ($ary1[$i] > $ary2[$j]) {
313 last; # can't go to outer :-(
314 }
315 $ary1[$i] += $ary2[$j];
316 }
cb1a09d0 317 # this is where that last takes me
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318 }
319
184e9718 320Whereas here's how a Perl programmer more comfortable with the idiom might
cb1a09d0 321do it:
4633a7c4 322
54310121 323 OUTER: foreach my $wid (@ary1) {
55497cff 324 INNER: foreach my $jet (@ary2) {
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325 next OUTER if $wid > $jet;
326 $wid += $jet;
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327 }
328 }
4633a7c4 329
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330See how much easier this is? It's cleaner, safer, and faster. It's
331cleaner because it's less noisy. It's safer because if code gets added
c07a80fd 332between the inner and outer loops later on, the new code won't be
5f05dabc 333accidentally executed. The C<next> explicitly iterates the other loop
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334rather than merely terminating the inner one. And it's faster because
335Perl executes a C<foreach> statement more rapidly than it would the
336equivalent C<for> loop.
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337
338=head2 Basic BLOCKs and Switch Statements
339
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340A BLOCK by itself (labeled or not) is semantically equivalent to a
341loop that executes once. Thus you can use any of the loop control
342statements in it to leave or restart the block. (Note that this is
343I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief
344C<do{}> blocks, which do I<NOT> count as loops.) The C<continue>
345block is optional.
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346
347The BLOCK construct is particularly nice for doing case
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348structures.
349
350 SWITCH: {
351 if (/^abc/) { $abc = 1; last SWITCH; }
352 if (/^def/) { $def = 1; last SWITCH; }
353 if (/^xyz/) { $xyz = 1; last SWITCH; }
354 $nothing = 1;
355 }
356
357There is no official switch statement in Perl, because there are
358already several ways to write the equivalent. In addition to the
359above, you could write
360
361 SWITCH: {
362 $abc = 1, last SWITCH if /^abc/;
363 $def = 1, last SWITCH if /^def/;
364 $xyz = 1, last SWITCH if /^xyz/;
365 $nothing = 1;
366 }
367
cb1a09d0 368(That's actually not as strange as it looks once you realize that you can
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369use loop control "operators" within an expression, That's just the normal
370C comma operator.)
371
372or
373
374 SWITCH: {
375 /^abc/ && do { $abc = 1; last SWITCH; };
376 /^def/ && do { $def = 1; last SWITCH; };
377 /^xyz/ && do { $xyz = 1; last SWITCH; };
378 $nothing = 1;
379 }
380
381or formatted so it stands out more as a "proper" switch statement:
382
383 SWITCH: {
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384 /^abc/ && do {
385 $abc = 1;
386 last SWITCH;
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387 };
388
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389 /^def/ && do {
390 $def = 1;
391 last SWITCH;
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392 };
393
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394 /^xyz/ && do {
395 $xyz = 1;
396 last SWITCH;
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397 };
398 $nothing = 1;
399 }
400
401or
402
403 SWITCH: {
404 /^abc/ and $abc = 1, last SWITCH;
405 /^def/ and $def = 1, last SWITCH;
406 /^xyz/ and $xyz = 1, last SWITCH;
407 $nothing = 1;
408 }
409
410or even, horrors,
411
412 if (/^abc/)
413 { $abc = 1 }
414 elsif (/^def/)
415 { $def = 1 }
416 elsif (/^xyz/)
417 { $xyz = 1 }
418 else
419 { $nothing = 1 }
420
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421
422A common idiom for a switch statement is to use C<foreach>'s aliasing to make
423a temporary assignment to $_ for convenient matching:
424
425 SWITCH: for ($where) {
426 /In Card Names/ && do { push @flags, '-e'; last; };
427 /Anywhere/ && do { push @flags, '-h'; last; };
428 /In Rulings/ && do { last; };
429 die "unknown value for form variable where: `$where'";
54310121 430 }
4633a7c4 431
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432Another interesting approach to a switch statement is arrange
433for a C<do> block to return the proper value:
434
435 $amode = do {
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436 if ($flag & O_RDONLY) { "r" }
437 elsif ($flag & O_WRONLY) { ($flag & O_APPEND) ? "a" : "w" }
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438 elsif ($flag & O_RDWR) {
439 if ($flag & O_CREAT) { "w+" }
c07a80fd 440 else { ($flag & O_APPEND) ? "a+" : "r+" }
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441 }
442 };
443
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444=head2 Goto
445
446Although not for the faint of heart, Perl does support a C<goto> statement.
447A loop's LABEL is not actually a valid target for a C<goto>;
448it's just the name of the loop. There are three forms: goto-LABEL,
449goto-EXPR, and goto-&NAME.
450
451The goto-LABEL form finds the statement labeled with LABEL and resumes
452execution there. It may not be used to go into any construct that
453requires initialization, such as a subroutine or a foreach loop. It
454also can't be used to go into a construct that is optimized away. It
455can be used to go almost anywhere else within the dynamic scope,
456including out of subroutines, but it's usually better to use some other
457construct such as last or die. The author of Perl has never felt the
458need to use this form of goto (in Perl, that is--C is another matter).
459
460The goto-EXPR form expects a label name, whose scope will be resolved
461dynamically. This allows for computed gotos per FORTRAN, but isn't
462necessarily recommended if you're optimizing for maintainability:
463
464 goto ("FOO", "BAR", "GLARCH")[$i];
465
466The goto-&NAME form is highly magical, and substitutes a call to the
467named subroutine for the currently running subroutine. This is used by
468AUTOLOAD() subroutines that wish to load another subroutine and then
469pretend that the other subroutine had been called in the first place
470(except that any modifications to @_ in the current subroutine are
471propagated to the other subroutine.) After the C<goto>, not even caller()
472will be able to tell that this routine was called first.
473
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474In almost all cases like this, it's usually a far, far better idea to use the
475structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of
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476resorting to a C<goto>. For certain applications, the catch and throw pair of
477C<eval{}> and die() for exception processing can also be a prudent approach.
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478
479=head2 PODs: Embedded Documentation
480
481Perl has a mechanism for intermixing documentation with source code.
c07a80fd 482While it's expecting the beginning of a new statement, if the compiler
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483encounters a line that begins with an equal sign and a word, like this
484
485 =head1 Here There Be Pods!
486
487Then that text and all remaining text up through and including a line
488beginning with C<=cut> will be ignored. The format of the intervening
54310121 489text is described in L<perlpod>.
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490
491This allows you to intermix your source code
492and your documentation text freely, as in
493
494 =item snazzle($)
495
54310121 496 The snazzle() function will behave in the most spectacular
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497 form that you can possibly imagine, not even excepting
498 cybernetic pyrotechnics.
499
500 =cut back to the compiler, nuff of this pod stuff!
501
502 sub snazzle($) {
503 my $thingie = shift;
504 .........
54310121 505 }
cb1a09d0 506
54310121 507Note that pod translators should look at only paragraphs beginning
184e9718 508with a pod directive (it makes parsing easier), whereas the compiler
54310121 509actually knows to look for pod escapes even in the middle of a
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510paragraph. This means that the following secret stuff will be
511ignored by both the compiler and the translators.
512
513 $a=3;
514 =secret stuff
515 warn "Neither POD nor CODE!?"
516 =cut back
517 print "got $a\n";
518
519You probably shouldn't rely upon the warn() being podded out forever.
520Not all pod translators are well-behaved in this regard, and perhaps
521the compiler will become pickier.
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522
523One may also use pod directives to quickly comment out a section
524of code.
525
526=head2 Plain Old Comments (Not!)
527
528Much like the C preprocessor, perl can process line directives. Using
529this, one can control perl's idea of filenames and line numbers in
530error or warning messages (especially for strings that are processed
531with eval()). The syntax for this mechanism is the same as for most
532C preprocessors: it matches the regular expression
4b094ceb 533C</^#\s*line\s+(\d+)\s*(?:\s"([^"]*)")?/> with C<$1> being the line
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534number for the next line, and C<$2> being the optional filename
535(specified within quotes).
536
537Here are some examples that you should be able to type into your command
538shell:
539
540 % perl
541 # line 200 "bzzzt"
542 # the `#' on the previous line must be the first char on line
543 die 'foo';
544 __END__
545 foo at bzzzt line 201.
54310121 546
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547 % perl
548 # line 200 "bzzzt"
549 eval qq[\n#line 2001 ""\ndie 'foo']; print $@;
550 __END__
551 foo at - line 2001.
54310121 552
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553 % perl
554 eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@;
555 __END__
556 foo at foo bar line 200.
54310121 557
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558 % perl
559 # line 345 "goop"
560 eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'";
561 print $@;
562 __END__
563 foo at goop line 345.
564
565=cut