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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
02c45c47 50C<myname> would function as a unary operator, so either C<or> or
54310121 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
ecca16b0 82 foreach EXPR
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83
84The C<if> and C<unless> modifiers have the expected semantics,
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85presuming you're a speaker of English. The C<foreach> modifier is an
86iterator: For each value in EXPR, it aliases $_ to the value and
87executes the statement. The C<while> and C<until> modifiers have the
88usual "while loop" semantics (conditional evaluated first), except
89when applied to a do-BLOCK (or to the now-deprecated do-SUBROUTINE
90statement), in which case the block executes once before the
91conditional is evaluated. This is so that you can write loops like:
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92
93 do {
4633a7c4 94 $line = <STDIN>;
a0d0e21e 95 ...
4633a7c4 96 } until $line eq ".\n";
a0d0e21e 97
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98See L<perlfunc/do>. Note also that the loop control statements described
99later will I<NOT> work in this construct, because modifiers don't take
100loop labels. Sorry. You can always put another block inside of it
101(for C<next>) or around it (for C<last>) to do that sort of thing.
102For next, just double the braces:
103
104 do {{
105 next if $x == $y;
106 # do something here
107 }} until $x++ > $z;
108
109For last, you have to be more elaborate:
110
111 LOOP: {
112 do {
113 last if $x = $y**2;
114 # do something here
115 } while $x++ <= $z;
116 }
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117
118=head2 Compound statements
119
120In Perl, a sequence of statements that defines a scope is called a block.
121Sometimes a block is delimited by the file containing it (in the case
122of a required file, or the program as a whole), and sometimes a block
123is delimited by the extent of a string (in the case of an eval).
124
125But generally, a block is delimited by curly brackets, also known as braces.
126We will call this syntactic construct a BLOCK.
127
128The following compound statements may be used to control flow:
129
130 if (EXPR) BLOCK
131 if (EXPR) BLOCK else BLOCK
132 if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
133 LABEL while (EXPR) BLOCK
134 LABEL while (EXPR) BLOCK continue BLOCK
135 LABEL for (EXPR; EXPR; EXPR) BLOCK
748a9306 136 LABEL foreach VAR (LIST) BLOCK
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137 LABEL BLOCK continue BLOCK
138
139Note that, unlike C and Pascal, these are defined in terms of BLOCKs,
140not statements. This means that the curly brackets are I<required>--no
141dangling statements allowed. If you want to write conditionals without
142curly brackets there are several other ways to do it. The following
143all do the same thing:
144
145 if (!open(FOO)) { die "Can't open $FOO: $!"; }
146 die "Can't open $FOO: $!" unless open(FOO);
147 open(FOO) or die "Can't open $FOO: $!"; # FOO or bust!
148 open(FOO) ? 'hi mom' : die "Can't open $FOO: $!";
149 # a bit exotic, that last one
150
5f05dabc 151The C<if> statement is straightforward. Because BLOCKs are always
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152bounded by curly brackets, there is never any ambiguity about which
153C<if> an C<else> goes with. If you use C<unless> in place of C<if>,
154the sense of the test is reversed.
155
156The C<while> statement executes the block as long as the expression is
157true (does not evaluate to the null string or 0 or "0"). The LABEL is
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158optional, and if present, consists of an identifier followed by a colon.
159The LABEL identifies the loop for the loop control statements C<next>,
160C<last>, and C<redo>. If the LABEL is omitted, the loop control statement
161refers to the innermost enclosing loop. This may include dynamically
162looking back your call-stack at run time to find the LABEL. Such
163desperate behavior triggers a warning if you use the B<-w> flag.
164
165If there is a C<continue> BLOCK, it is always executed just before the
166conditional is about to be evaluated again, just like the third part of a
167C<for> loop in C. Thus it can be used to increment a loop variable, even
168when the loop has been continued via the C<next> statement (which is
169similar to the C C<continue> statement).
170
171=head2 Loop Control
172
173The C<next> command is like the C<continue> statement in C; it starts
174the next iteration of the loop:
175
176 LINE: while (<STDIN>) {
177 next LINE if /^#/; # discard comments
178 ...
179 }
180
181The C<last> command is like the C<break> statement in C (as used in
182loops); it immediately exits the loop in question. The
183C<continue> block, if any, is not executed:
184
185 LINE: while (<STDIN>) {
186 last LINE if /^$/; # exit when done with header
187 ...
188 }
189
190The C<redo> command restarts the loop block without evaluating the
191conditional again. The C<continue> block, if any, is I<not> executed.
192This command is normally used by programs that want to lie to themselves
193about what was just input.
194
195For example, when processing a file like F</etc/termcap>.
196If your input lines might end in backslashes to indicate continuation, you
197want to skip ahead and get the next record.
198
199 while (<>) {
200 chomp;
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201 if (s/\\$//) {
202 $_ .= <>;
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203 redo unless eof();
204 }
205 # now process $_
54310121 206 }
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207
208which is Perl short-hand for the more explicitly written version:
209
54310121 210 LINE: while (defined($line = <ARGV>)) {
4633a7c4 211 chomp($line);
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212 if ($line =~ s/\\$//) {
213 $line .= <ARGV>;
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214 redo LINE unless eof(); # not eof(ARGV)!
215 }
216 # now process $line
54310121 217 }
4633a7c4 218
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219Note that if there were a C<continue> block on the above code, it would get
220executed even on discarded lines. This is often used to reset line counters
221or C<?pat?> one-time matches.
4633a7c4 222
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223 # inspired by :1,$g/fred/s//WILMA/
224 while (<>) {
225 ?(fred)? && s//WILMA $1 WILMA/;
226 ?(barney)? && s//BETTY $1 BETTY/;
227 ?(homer)? && s//MARGE $1 MARGE/;
228 } continue {
229 print "$ARGV $.: $_";
230 close ARGV if eof(); # reset $.
231 reset if eof(); # reset ?pat?
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232 }
233
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234If the word C<while> is replaced by the word C<until>, the sense of the
235test is reversed, but the conditional is still tested before the first
236iteration.
237
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238The loop control statements don't work in an C<if> or C<unless>, since
239they aren't loops. You can double the braces to make them such, though.
240
241 if (/pattern/) {{
242 next if /fred/;
243 next if /barney/;
244 # so something here
245 }}
246
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247The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer
248available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>.
4633a7c4 249
cb1a09d0 250=head2 For Loops
a0d0e21e 251
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252Perl's C-style C<for> loop works exactly like the corresponding C<while> loop;
253that means that this:
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254
255 for ($i = 1; $i < 10; $i++) {
256 ...
257 }
258
cb1a09d0 259is the same as this:
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260
261 $i = 1;
262 while ($i < 10) {
263 ...
264 } continue {
265 $i++;
266 }
267
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268(There is one minor difference: The first form implies a lexical scope
269for variables declared with C<my> in the initialization expression.)
270
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271Besides the normal array index looping, C<for> can lend itself
272to many other interesting applications. Here's one that avoids the
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273problem you get into if you explicitly test for end-of-file on
274an interactive file descriptor causing your program to appear to
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275hang.
276
277 $on_a_tty = -t STDIN && -t STDOUT;
278 sub prompt { print "yes? " if $on_a_tty }
279 for ( prompt(); <STDIN>; prompt() ) {
280 # do something
54310121 281 }
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282
283=head2 Foreach Loops
284
4633a7c4 285The C<foreach> loop iterates over a normal list value and sets the
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286variable VAR to be each element of the list in turn. If the variable
287is preceded with the keyword C<my>, then it is lexically scoped, and
288is therefore visible only within the loop. Otherwise, the variable is
289implicitly local to the loop and regains its former value upon exiting
290the loop. If the variable was previously declared with C<my>, it uses
291that variable instead of the global one, but it's still localized to
292the loop. (Note that a lexically scoped variable can cause problems
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293if you have subroutine or format declarations within the loop which
294refer to it.)
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295
296The C<foreach> keyword is actually a synonym for the C<for> keyword, so
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297you can use C<foreach> for readability or C<for> for brevity. (Or because
298the Bourne shell is more familiar to you than I<csh>, so writing C<for>
299comes more naturally.) If VAR is omitted, $_ is set to each value.
300If any element of LIST is an lvalue, you can modify it by modifying VAR
301inside the loop. That's because the C<foreach> loop index variable is
302an implicit alias for each item in the list that you're looping over.
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303
304If any part of LIST is an array, C<foreach> will get very confused if
305you add or remove elements within the loop body, for example with
306C<splice>. So don't do that.
307
308C<foreach> probably won't do what you expect if VAR is a tied or other
309special variable. Don't do that either.
4633a7c4 310
748a9306 311Examples:
a0d0e21e 312
4633a7c4 313 for (@ary) { s/foo/bar/ }
a0d0e21e 314
55497cff 315 foreach my $elem (@elements) {
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316 $elem *= 2;
317 }
318
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319 for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') {
320 print $count, "\n"; sleep(1);
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321 }
322
323 for (1..15) { print "Merry Christmas\n"; }
324
4633a7c4 325 foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) {
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326 print "Item: $item\n";
327 }
328
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329Here's how a C programmer might code up a particular algorithm in Perl:
330
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331 for (my $i = 0; $i < @ary1; $i++) {
332 for (my $j = 0; $j < @ary2; $j++) {
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333 if ($ary1[$i] > $ary2[$j]) {
334 last; # can't go to outer :-(
335 }
336 $ary1[$i] += $ary2[$j];
337 }
cb1a09d0 338 # this is where that last takes me
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339 }
340
184e9718 341Whereas here's how a Perl programmer more comfortable with the idiom might
cb1a09d0 342do it:
4633a7c4 343
54310121 344 OUTER: foreach my $wid (@ary1) {
55497cff 345 INNER: foreach my $jet (@ary2) {
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346 next OUTER if $wid > $jet;
347 $wid += $jet;
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348 }
349 }
4633a7c4 350
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351See how much easier this is? It's cleaner, safer, and faster. It's
352cleaner because it's less noisy. It's safer because if code gets added
c07a80fd 353between the inner and outer loops later on, the new code won't be
5f05dabc 354accidentally executed. The C<next> explicitly iterates the other loop
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355rather than merely terminating the inner one. And it's faster because
356Perl executes a C<foreach> statement more rapidly than it would the
357equivalent C<for> loop.
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358
359=head2 Basic BLOCKs and Switch Statements
360
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361A BLOCK by itself (labeled or not) is semantically equivalent to a
362loop that executes once. Thus you can use any of the loop control
363statements in it to leave or restart the block. (Note that this is
364I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief
365C<do{}> blocks, which do I<NOT> count as loops.) The C<continue>
366block is optional.
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367
368The BLOCK construct is particularly nice for doing case
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369structures.
370
371 SWITCH: {
372 if (/^abc/) { $abc = 1; last SWITCH; }
373 if (/^def/) { $def = 1; last SWITCH; }
374 if (/^xyz/) { $xyz = 1; last SWITCH; }
375 $nothing = 1;
376 }
377
378There is no official switch statement in Perl, because there are
379already several ways to write the equivalent. In addition to the
380above, you could write
381
382 SWITCH: {
383 $abc = 1, last SWITCH if /^abc/;
384 $def = 1, last SWITCH if /^def/;
385 $xyz = 1, last SWITCH if /^xyz/;
386 $nothing = 1;
387 }
388
cb1a09d0 389(That's actually not as strange as it looks once you realize that you can
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390use loop control "operators" within an expression, That's just the normal
391C comma operator.)
392
393or
394
395 SWITCH: {
396 /^abc/ && do { $abc = 1; last SWITCH; };
397 /^def/ && do { $def = 1; last SWITCH; };
398 /^xyz/ && do { $xyz = 1; last SWITCH; };
399 $nothing = 1;
400 }
401
402or formatted so it stands out more as a "proper" switch statement:
403
404 SWITCH: {
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405 /^abc/ && do {
406 $abc = 1;
407 last SWITCH;
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408 };
409
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410 /^def/ && do {
411 $def = 1;
412 last SWITCH;
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413 };
414
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415 /^xyz/ && do {
416 $xyz = 1;
417 last SWITCH;
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418 };
419 $nothing = 1;
420 }
421
422or
423
424 SWITCH: {
425 /^abc/ and $abc = 1, last SWITCH;
426 /^def/ and $def = 1, last SWITCH;
427 /^xyz/ and $xyz = 1, last SWITCH;
428 $nothing = 1;
429 }
430
431or even, horrors,
432
433 if (/^abc/)
434 { $abc = 1 }
435 elsif (/^def/)
436 { $def = 1 }
437 elsif (/^xyz/)
438 { $xyz = 1 }
439 else
440 { $nothing = 1 }
441
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442A common idiom for a switch statement is to use C<foreach>'s aliasing to make
443a temporary assignment to $_ for convenient matching:
444
445 SWITCH: for ($where) {
446 /In Card Names/ && do { push @flags, '-e'; last; };
447 /Anywhere/ && do { push @flags, '-h'; last; };
448 /In Rulings/ && do { last; };
449 die "unknown value for form variable where: `$where'";
54310121 450 }
4633a7c4 451
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452Another interesting approach to a switch statement is arrange
453for a C<do> block to return the proper value:
454
455 $amode = do {
5a964f20 456 if ($flag & O_RDONLY) { "r" } # XXX: isn't this 0?
54310121 457 elsif ($flag & O_WRONLY) { ($flag & O_APPEND) ? "a" : "w" }
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458 elsif ($flag & O_RDWR) {
459 if ($flag & O_CREAT) { "w+" }
c07a80fd 460 else { ($flag & O_APPEND) ? "a+" : "r+" }
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461 }
462 };
463
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464Or
465
466 print do {
467 ($flags & O_WRONLY) ? "write-only" :
468 ($flags & O_RDWR) ? "read-write" :
469 "read-only";
470 };
471
472Or if you are certainly that all the C<&&> clauses are true, you can use
473something like this, which "switches" on the value of the
474HTTP_USER_AGENT envariable.
475
476 #!/usr/bin/perl
477 # pick out jargon file page based on browser
478 $dir = 'http://www.wins.uva.nl/~mes/jargon';
479 for ($ENV{HTTP_USER_AGENT}) {
480 $page = /Mac/ && 'm/Macintrash.html'
481 || /Win(dows )?NT/ && 'e/evilandrude.html'
482 || /Win|MSIE|WebTV/ && 'm/MicroslothWindows.html'
483 || /Linux/ && 'l/Linux.html'
484 || /HP-UX/ && 'h/HP-SUX.html'
485 || /SunOS/ && 's/ScumOS.html'
486 || 'a/AppendixB.html';
487 }
488 print "Location: $dir/$page\015\012\015\012";
489
490That kind of switch statement only works when you know the C<&&> clauses
491will be true. If you don't, the previous C<?:> example should be used.
492
493You might also consider writing a hash instead of synthesizing a switch
494statement.
495
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496=head2 Goto
497
498Although not for the faint of heart, Perl does support a C<goto> statement.
499A loop's LABEL is not actually a valid target for a C<goto>;
500it's just the name of the loop. There are three forms: goto-LABEL,
501goto-EXPR, and goto-&NAME.
502
503The goto-LABEL form finds the statement labeled with LABEL and resumes
504execution there. It may not be used to go into any construct that
505requires initialization, such as a subroutine or a foreach loop. It
506also can't be used to go into a construct that is optimized away. It
507can be used to go almost anywhere else within the dynamic scope,
508including out of subroutines, but it's usually better to use some other
509construct such as last or die. The author of Perl has never felt the
510need to use this form of goto (in Perl, that is--C is another matter).
511
512The goto-EXPR form expects a label name, whose scope will be resolved
513dynamically. This allows for computed gotos per FORTRAN, but isn't
514necessarily recommended if you're optimizing for maintainability:
515
516 goto ("FOO", "BAR", "GLARCH")[$i];
517
518The goto-&NAME form is highly magical, and substitutes a call to the
519named subroutine for the currently running subroutine. This is used by
520AUTOLOAD() subroutines that wish to load another subroutine and then
521pretend that the other subroutine had been called in the first place
522(except that any modifications to @_ in the current subroutine are
523propagated to the other subroutine.) After the C<goto>, not even caller()
524will be able to tell that this routine was called first.
525
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526In almost all cases like this, it's usually a far, far better idea to use the
527structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of
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528resorting to a C<goto>. For certain applications, the catch and throw pair of
529C<eval{}> and die() for exception processing can also be a prudent approach.
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530
531=head2 PODs: Embedded Documentation
532
533Perl has a mechanism for intermixing documentation with source code.
c07a80fd 534While it's expecting the beginning of a new statement, if the compiler
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535encounters a line that begins with an equal sign and a word, like this
536
537 =head1 Here There Be Pods!
538
539Then that text and all remaining text up through and including a line
540beginning with C<=cut> will be ignored. The format of the intervening
54310121 541text is described in L<perlpod>.
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542
543This allows you to intermix your source code
544and your documentation text freely, as in
545
546 =item snazzle($)
547
54310121 548 The snazzle() function will behave in the most spectacular
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549 form that you can possibly imagine, not even excepting
550 cybernetic pyrotechnics.
551
552 =cut back to the compiler, nuff of this pod stuff!
553
554 sub snazzle($) {
555 my $thingie = shift;
556 .........
54310121 557 }
cb1a09d0 558
54310121 559Note that pod translators should look at only paragraphs beginning
184e9718 560with a pod directive (it makes parsing easier), whereas the compiler
54310121 561actually knows to look for pod escapes even in the middle of a
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562paragraph. This means that the following secret stuff will be
563ignored by both the compiler and the translators.
564
565 $a=3;
566 =secret stuff
567 warn "Neither POD nor CODE!?"
568 =cut back
569 print "got $a\n";
570
571You probably shouldn't rely upon the warn() being podded out forever.
572Not all pod translators are well-behaved in this regard, and perhaps
573the compiler will become pickier.
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574
575One may also use pod directives to quickly comment out a section
576of code.
577
578=head2 Plain Old Comments (Not!)
579
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580Much like the C preprocessor, Perl can process line directives. Using
581this, one can control Perl's idea of filenames and line numbers in
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582error or warning messages (especially for strings that are processed
583with eval()). The syntax for this mechanism is the same as for most
584C preprocessors: it matches the regular expression
4b094ceb 585C</^#\s*line\s+(\d+)\s*(?:\s"([^"]*)")?/> with C<$1> being the line
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586number for the next line, and C<$2> being the optional filename
587(specified within quotes).
588
589Here are some examples that you should be able to type into your command
590shell:
591
592 % perl
593 # line 200 "bzzzt"
594 # the `#' on the previous line must be the first char on line
595 die 'foo';
596 __END__
597 foo at bzzzt line 201.
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599 % perl
600 # line 200 "bzzzt"
601 eval qq[\n#line 2001 ""\ndie 'foo']; print $@;
602 __END__
603 foo at - line 2001.
54310121 604
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605 % perl
606 eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@;
607 __END__
608 foo at foo bar line 200.
54310121 609
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610 % perl
611 # line 345 "goop"
612 eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'";
613 print $@;
614 __END__
615 foo at goop line 345.
616
617=cut