File: perltrap.html

package info (click to toggle)
perl-doc-html 5.26.0-4
  • links: PTS, VCS
  • area: main
  • in suites: sid
  • size: 39,400 kB
  • sloc: xml: 36; makefile: 7
file content (848 lines) | stat: -rw-r--r-- 33,313 bytes parent folder | download
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
  <title>perltrap - perldoc.perl.org</title>
  <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
  <meta http-equiv="Content-Language" content="en-gb">
  <link rel="search" type="application/opensearchdescription+xml" title="Search perldoc.perl.org" href="/static/search.xml"/>
  <link href="static/css-20100830.css" rel="stylesheet" rev="stylesheet" type="text/css" media="screen">
  <link href="static/exploreperl.css" rel="stylesheet" rev="stylesheet" type="text/css">
</head>

<body onLoad="perldoc.startup();" onPageShow="if (event.persisted) perldoc.startup();">
    <div id="page">
      
      <div id="header">
	<div id="homepage_link">
	  <a href="index.html"></a>
	</div>
	<div id="strapline">
	  Perl Programming Documentation
	</div>
	<div id="download_link" class="download">
	  <a href="http://www.perl.org/get.html">Download Perl</a>
	</div>
	<div id="explore_link" class="download">
	  <a id="explore_anchor" href="#">Explore</a>
	</div>
      </div>
      
      <div id="body">
        <div id="left_column">
          <div class="side_group">
            
	    <div class="side_panel doc_panel">
              <p>Manual</p>
              <ul>
                <li><a href="index-overview.html">Overview</a>
                <li><a href="index-tutorials.html">Tutorials</a>
                <li><a href="index-faq.html">FAQs</a>
                <li><a href="index-history.html">History / Changes</a>
                <li><a href="index-licence.html">License</a>
              </ul>
            </div>
            <div class="side_panel doc_panel">
              <p>Reference</p>
              <ul>
                <li><a href="index-language.html">Language</a>
                <li><a href="index-functions.html">Functions</a>
                <li><a href="perlop.html">Operators</a>
                <li><a href="perlvar.html">Special Variables</a>
                <li><a href="index-pragmas.html">Pragmas</a>
                <li><a href="index-utilities.html">Utilities</a>
                <li><a href="index-internals.html">Internals</a>
                <li><a href="index-platforms.html">Platform Specific</a>
              </ul>
            </div>
            <div class="side_panel doc_panel">
              <p>Modules</p>
              <ul>
		<li>
		
                
                  
		    
		  
		
                  
		    
		  
		
                  
		    
		  
		
                  
		    
		  
		
                  
		    
		  
		
                  
		    
		  
		
                  
		    
		  
		
                  
		    
		  
		
                  
		    
		  
		
                  
		
                  
		
                  
		    
		  
		
                  
		    
		  
		
                  
		    
		  
		
                  
		    
		  
		
                  
		    
		  
		
                  
		
                  
		
                  
		    
		  
		
                  
		    
		  
		
                  
		    
		  
		
                  
		
                  
		
                  
		    
		  
		
                  
		
                  
		
		
                    <a href="index-modules-A.html">A</a>
                    
                      
                        &bull;
                      
                    
                
                    <a href="index-modules-B.html">B</a>
                    
                      
                        &bull;
                      
                    
                
                    <a href="index-modules-C.html">C</a>
                    
                      
                        &bull;
                      
                    
                
                    <a href="index-modules-D.html">D</a>
                    
                      
                        &bull;
                      
                    
                
                    <a href="index-modules-E.html">E</a>
                    
                      
                        <li>
                      
                    
                
                    <a href="index-modules-F.html">F</a>
                    
                      
                        &bull;
                      
                    
                
                    <a href="index-modules-G.html">G</a>
                    
                      
                        &bull;
                      
                    
                
                    <a href="index-modules-H.html">H</a>
                    
                      
                        &bull;
                      
                    
                
                    <a href="index-modules-I.html">I</a>
                    
                      
                        &bull;
                      
                    
                
                    <a href="index-modules-L.html">L</a>
                    
                      
                        <li>
                      
                    
                
                    <a href="index-modules-M.html">M</a>
                    
                      
                        &bull;
                      
                    
                
                    <a href="index-modules-N.html">N</a>
                    
                      
                        &bull;
                      
                    
                
                    <a href="index-modules-O.html">O</a>
                    
                      
                        &bull;
                      
                    
                
                    <a href="index-modules-P.html">P</a>
                    
                      
                        &bull;
                      
                    
                
                    <a href="index-modules-S.html">S</a>
                    
                      
                        <li>
                      
                    
                
                    <a href="index-modules-T.html">T</a>
                    
                      
                        &bull;
                      
                    
                
                    <a href="index-modules-U.html">U</a>
                    
                      
                        &bull;
                      
                    
                
                    <a href="index-modules-X.html">X</a>
                    
                
              </ul>
            </div>
            
	      <div class="side_panel doc_panel">
		<p>Tools</p>
		<ul>
		  <li><a href="preferences.html">Preferences</a>
		</ul>
	      </div>
            
          </div>
        </div>
        <div id="centre_column">
          <div id="content_header">
            <div id="title_bar">
              <div id="page_name">
                <h1>perltrap</h1>
              </div>
              <div id="perl_version">
                Perl 5 version 26.0 documentation
              </div>
              <div class="page_links" id="page_links_top">
                <a href="#" onClick="toolbar.goToTop();return false;">Go to top</a>
		
              </div>
	      <div class="page_links" id="page_links_bottom">
		
                  <a href="#" id="page_index_toggle">Show page index</a> &bull;
		
                <a href="#" id="recent_pages_toggle">Show recent pages</a>		
	      </div>
	      <div id="search_form">
		<form action="search.html" method="GET" id="search">
		  <input type="text" name="q" id="search_box" alt="Search">
		</form>
	      </div>
            </div>
            <div id="breadcrumbs">
                
    <a href="index.html">Home</a> &gt;
    
      
        <a href="index-tutorials.html">Tutorials</a> &gt;
      
    
    perltrap
  

            </div>
          </div>
          <div id="content_body">
	    <!--[if lt IE 7]>
 <div class="noscript">
   <p>
     <strong>It looks like you're using Internet Explorer 6. This is a very old
     browser which does not offer full support for modern websites.</strong>
   </p>
   <p>
     Unfortunately this means that this website will not work on
     your computer.
   </p>
   <p>
     Don't miss out though! To view the site (and get a better experience from
     many other websites), simply upgrade to
     <a href="http://www.microsoft.com/windows/Internet-explorer/default.aspx">Internet
Explorer 8</a>
     or download an alternative browser such as
     <a href="http://www.mozilla.com/en-US/firefox/firefox.html">Firefox</a>,
     <a href="http://www.apple.com/safari/download/">Safari</a>, or
     <a href="http://www.google.co.uk/chrome">Google Chrome</a>.
   </p>
   <p>
     All of these browsers are free. If you're using a PC at work, you may
     need to contact your IT administrator.
   </p>
 </div>
<![endif]-->
	    <noscript>
	      <div class="noscript">
	      <p>
                <strong>Please note: Many features of this site require JavaScript. You appear to have JavaScript disabled,
	        or are running a non-JavaScript capable web browser.</strong>
	      </p>
	      <p>
		To get the best experience, please enable JavaScript or download a modern web browser such as <a href="http://www.microsoft.com/windows/Internet-explorer/default.aspx">Internet Explorer 8</a>, <a href="http://www.mozilla.com/en-US/firefox/firefox.html">Firefox</a>, <a href="http://www.apple.com/safari/download/">Safari</a>, or <a href="http://www.google.co.uk/chrome">Google Chrome</a>.
              </p>
	      </div>
	    </noscript>

	    <div id="recent_pages" class="hud_container">
	      <div id="recent_pages_header" class="hud_header">
		<div id="recent_pages_close" class="hud_close"><a href="#" onClick="recentPages.hide();return false;"></a></div>
		<div id="recent_pages_title" class="hud_title"><span class="hud_span_top">Recently read</span></div>
		<div id="recent_pages_topright" class="hud_topright"></div>
	      </div>
	      <div id="recent_pages_content" class="hud_content">
	      </div>
	      <div id="recent_pages_footer" class="hud_footer">
		<div id="recent_pages_bottomleft" class="hud_bottomleft"></div>
		<div id="recent_pages_bottom" class="hud_bottom"><span class="hud_span_bottom"></span></div>
		<div id="recent_pages_resize" class="hud_resize"></div>
	      </div>
	    </div>
  
	    <div id="from_search"></div>
            <h1>perltrap</h1>


  <!--    -->
<ul><li><a href="#NAME">NAME</a><li><a href="#DESCRIPTION">DESCRIPTION</a><ul><li><a href="#Awk-Traps">Awk Traps</a><li><a href="#C%2fC%2b%2b-Traps">C/C++ Traps</a><li><a href="#JavaScript-Traps">JavaScript Traps</a><li><a href="#Sed-Traps">Sed Traps</a><li><a href="#Shell-Traps">Shell Traps</a><li><a href="#Perl-Traps">Perl Traps</a></ul></ul><a name="NAME"></a><h1>NAME</h1>
<p>perltrap - Perl traps for the unwary</p>
<a name="DESCRIPTION"></a><h1>DESCRIPTION</h1>
<p>The biggest trap of all is forgetting to <code class="inline"><a class="l_k" href="functions/use.html">use</a> <span class="w">warnings</span></code>
 or use the <b>-w</b>
switch; see <a href="warnings.html">warnings</a> and <a href="perlrun.html">perlrun</a>. The second biggest trap is not
making your entire program runnable under <code class="inline"><a class="l_k" href="functions/use.html">use</a> <span class="w">strict</span></code>
.  The third biggest
trap is not reading the list of changes in this version of Perl; see
<a href="perldelta.html">perldelta</a>.</p>
<a name="Awk-Traps"></a><h2>Awk Traps</h2>
<p>Accustomed <b>awk</b> users should take special note of the following:</p>
<ul>
<li>
<p>A Perl program executes only once, not once for each input line.  You can
do an implicit loop with <code class="inline">-<span class="w">n</span></code>
 or <code class="inline">-p</code>
.</p>
</li>
<li>
<p>The English module, loaded via</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/use.html">use</a> <span class="w">English</span><span class="sc">;</span></li></ol></pre><p>allows you to refer to special variables (like <code class="inline"><span class="i">$/</span></code>
) with names (like
$RS), as though they were in <b>awk</b>; see <a href="perlvar.html">perlvar</a> for details.</p>
</li>
<li>
<p>Semicolons are required after all simple statements in Perl (except
at the end of a block).  Newline is not a statement delimiter.</p>
</li>
<li>
<p>Curly brackets are required on <code class="inline">if</code>
s and <code class="inline">while</code>
s.</p>
</li>
<li>
<p>Variables begin with "$", "@" or "%" in Perl.</p>
</li>
<li>
<p>Arrays index from 0.  Likewise string positions in substr() and
index().</p>
</li>
<li>
<p>You have to decide whether your array has numeric or string indices.</p>
</li>
<li>
<p>Hash values do not spring into existence upon mere reference.</p>
</li>
<li>
<p>You have to decide whether you want to use string or numeric
comparisons.</p>
</li>
<li>
<p>Reading an input line does not split it for you.  You get to split it
to an array yourself.  And the split() operator has different
arguments than <b>awk</b>'s.</p>
</li>
<li>
<p>The current input line is normally in $_, not $0.  It generally does
not have the newline stripped.  ($0 is the name of the program
executed.)  See <a href="perlvar.html">perlvar</a>.</p>
</li>
<li>
<p>$&lt;<i>digit</i>&gt; does not refer to fields--it refers to substrings matched
by the last match pattern.</p>
</li>
<li>
<p>The print() statement does not add field and record separators unless
you set <code class="inline"><span class="i">$,</span></code>
 and <code class="inline"><span class="i">$\</span></code>
.  You can set $OFS and $ORS if you're using
the English module.</p>
</li>
<li>
<p>You must open your files before you print to them.</p>
</li>
<li>
<p>The range operator is "..", not comma.  The comma operator works as in
C.</p>
</li>
<li>
<p>The match operator is "=~", not "~".  ("~" is the one's complement
operator, as in C.)</p>
</li>
<li>
<p>The exponentiation operator is "**", not "^".  "^" is the XOR
operator, as in C.  (You know, one could get the feeling that <b>awk</b> is
basically incompatible with C.)</p>
</li>
<li>
<p>The concatenation operator is ".", not the null string.  (Using the
null string would render <code class="inline"><span class="q">/pat/</span> /<span class="w">pat</span>/</code>
 unparsable, because the third slash
would be interpreted as a division operator--the tokenizer is in fact
slightly context sensitive for operators like "/", "?", and "&gt;".
And in fact, "." itself can be the beginning of a number.)</p>
</li>
<li>
<p>The <code class="inline"><a class="l_k" href="functions/next.html">next</a></code>, <code class="inline"><a class="l_k" href="functions/exit.html">exit</a></code>, and <code class="inline"><a class="l_k" href="functions/continue.html">continue</a></code> keywords work differently.</p>
</li>
<li>
<p>The following variables work differently:</p>
<pre class="verbatim"><ol><li>      Awk	Perl</li><li>      ARGC	scalar @ARGV (compare with $#ARGV)</li><li>      ARGV[0]	$0</li><li>      FILENAME	$ARGV</li><li>      FNR	$. - something</li><li>      FS	(whatever you like)</li><li>      NF	$#Fld, or some such</li><li>      NR	$.</li><li>      OFMT	$#</li><li>      OFS	$,</li><li>      ORS	$\</li><li>      RLENGTH	length($&amp;)</li><li>      RS	$/</li><li>      RSTART	length($`)</li><li>      SUBSEP	$;</li></ol></pre></li>
<li>
<p>You cannot set $RS to a pattern, only a string.</p>
</li>
<li>
<p>When in doubt, run the <b>awk</b> construct through <b>a2p</b> and see what it
gives you.</p>
</li>
</ul>
<a name="C%2fC%2b%2b-Traps"></a><h2>C/C++ Traps</h2>
<p>Cerebral C and C++ programmers should take note of the following:</p>
<ul>
<li>
<p>Curly brackets are required on <code class="inline">if</code>
's and <code class="inline">while</code>
's.</p>
</li>
<li>
<p>You must use <code class="inline">elsif</code>
 rather than <code class="inline">else if</code>
.</p>
</li>
<li>
<p>The <code class="inline"><span class="w">break</span></code>
 and <code class="inline"><a class="l_k" href="functions/continue.html">continue</a></code> keywords from C become in Perl <code class="inline"><a class="l_k" href="functions/last.html">last</a></code>
and <code class="inline"><a class="l_k" href="functions/next.html">next</a></code>, respectively.  Unlike in C, these do <i>not</i> work within a
<code class="inline"><a class="l_k" href="functions/do.html">do</a> <span class="s">{</span> <span class="s">}</span> while</code>
 construct.  See <a href="perlsyn.html#Loop-Control">Loop Control in perlsyn</a>.</p>
</li>
<li>
<p>The switch statement is called <code class="inline">given</code>
/<code class="inline">when</code>
 and only available in
perl 5.10 or newer.  See <a href="perlsyn.html#Switch-Statements">Switch Statements in perlsyn</a>.</p>
</li>
<li>
<p>Variables begin with "$", "@" or "%" in Perl.</p>
</li>
<li>
<p>Comments begin with "#", not "/*" or "//".  Perl may interpret C/C++
comments as division operators, unterminated regular expressions or
the defined-or operator.</p>
</li>
<li>
<p>You can't take the address of anything, although a similar operator
in Perl is the backslash, which creates a reference.</p>
</li>
<li>
<p><code class="inline"><span class="w">ARGV</span></code>
 must be capitalized.  <code class="inline"><span class="i">$ARGV</span>[<span class="n">0</span>]</code>
 is C's <code class="inline"><span class="w">argv</span><span class="s">[</span><span class="n">1</span><span class="s">]</span></code>
, and <code class="inline"><span class="w">argv</span><span class="s">[</span><span class="n">0</span><span class="s">]</span></code>

ends up in <code class="inline"><span class="i">$0</span></code>
.</p>
</li>
<li>
<p>System calls such as link(), unlink(), rename(), etc. return nonzero for
success, not 0. (system(), however, returns zero for success.)</p>
</li>
<li>
<p>Signal handlers deal with signal names, not numbers.  Use <code class="inline"><a class="l_k" href="functions/kill.html">kill</a> -l</code>

to find their names on your system.</p>
</li>
</ul>
<a name="JavaScript-Traps"></a><h2>JavaScript Traps</h2>
<p>Judicious JavaScript programmers should take note of the following:</p>
<ul>
<li>
<p>In Perl, binary <code class="inline">+</code>
 is always addition.  <code class="inline"><span class="i">$string1</span> + <span class="i">$string2</span></code>
 converts
both strings to numbers and then adds them.  To concatenate two strings,
use the <code class="inline">.</code> operator.</p>
</li>
<li>
<p>The <code class="inline">+</code>
 unary operator doesn't do anything in Perl.  It exists to avoid
syntactic ambiguities.</p>
</li>
<li>
<p>Unlike <code class="inline"><a class="l_k" href="functions/for.html">for...in</a></code>, Perl's <code class="inline">for</code>
 (also spelled <code class="inline">foreach</code>
) does not allow
the left-hand side to be an arbitrary expression.  It must be a variable:</p>
<pre class="verbatim"><ol><li>   <a class="l_k" href="functions/for.html">for</a> <a class="l_k" href="functions/my.html">my</a> <span class="i">$variable</span> <span class="s">(</span><a class="l_k" href="functions/keys.html">keys</a> <span class="i">%hash</span><span class="s">)</span> <span class="s">{</span></li><li>	...</li><li>   <span class="s">}</span></li></ol></pre><p>Furthermore, don't forget the <code class="inline"><a class="l_k" href="functions/keys.html">keys</a></code> in there, as
<code class="inline"><a class="l_k" href="functions/foreach.html">foreach</a> <a class="l_k" href="functions/my.html">my</a> <span class="i">$kv</span> <span class="s">(</span><span class="i">%hash</span><span class="s">)</span> <span class="s">{</span><span class="s">}</span></code>
 iterates over the keys and values, and is
generally not useful ($kv would be a key, then a value, and so on).</p>
</li>
<li>
<p>To iterate over the indices of an array, use <code class="inline"><a class="l_k" href="functions/foreach.html">foreach</a> <a class="l_k" href="functions/my.html">my</a> <span class="i">$i</span> <span class="s">(</span><span class="n">0</span> .. <span class="i">$#array</span><span class="s">)</span>
<span class="s">{</span><span class="s">}</span></code>
.  <code class="inline"><a class="l_k" href="functions/foreach.html">foreach</a> <a class="l_k" href="functions/my.html">my</a> <span class="i">$v</span> <span class="s">(</span><span class="i">@array</span><span class="s">)</span> <span class="s">{</span><span class="s">}</span></code>
 iterates over the values.</p>
</li>
<li>
<p>Perl requires braces following <code class="inline">if</code>
, <code class="inline">while</code>
, <code class="inline">foreach</code>
, etc.</p>
</li>
<li>
<p>In Perl, <code class="inline">else if</code>
 is spelled <code class="inline">elsif</code>
.</p>
</li>
<li>
<p><code class="inline"><span class="q">? :</span></code>
 has higher precedence than assignment.  In JavaScript, one can
write:</p>
<pre class="verbatim"><ol><li>    <span class="w">condition</span> ? <span class="i">do_something</span><span class="s">(</span><span class="s">)</span> <span class="co">:</span> <span class="w">variable</span> = <span class="n">3</span></li></ol></pre><p>and the variable is only assigned if the condition is false.  In Perl, you
need parentheses:</p>
<pre class="verbatim"><ol><li>    <span class="i">$condition</span> ? <span class="i">do_something</span><span class="s">(</span><span class="s">)</span> <span class="co">:</span> <span class="s">(</span><span class="i">$variable</span> = <span class="n">3</span><span class="s">)</span><span class="sc">;</span></li></ol></pre><p>Or just use <code class="inline">if</code>
.</p>
</li>
<li>
<p>Perl requires semicolons to separate statements.</p>
</li>
<li>
<p>Variables declared with <code class="inline"><a class="l_k" href="functions/my.html">my</a></code> only affect code <i>after</i> the declaration.
You cannot write <code class="inline"><span class="i">$x</span> = <span class="n">1</span><span class="sc">;</span> <a class="l_k" href="functions/my.html">my</a> <span class="i">$x</span><span class="sc">;</span></code>
 and expect the first assignment to
affect the same variable.  It will instead assign to an <code class="inline"><span class="i">$x</span></code>
 declared
previously in an outer scope, or to a global variable.</p>
<p>Note also that the variable is not visible until the following
<i>statement</i>.  This means that in <code class="inline"><a class="l_k" href="functions/my.html">my</a> <span class="i">$x</span> = <span class="n">1</span> + <span class="i">$x</span></code>
 the second $x refers
to one declared previously.</p>
</li>
<li>
<p><code class="inline"><a class="l_k" href="functions/my.html">my</a></code> variables are scoped to the current block, not to the current
function.  If you write <code class="inline"><span class="s">{</span><a class="l_k" href="functions/my.html">my</a> <span class="i">$x</span><span class="sc">;</span><span class="s">}</span> <span class="i">$x</span><span class="sc">;</span></code>
, the second <code class="inline"><span class="i">$x</span></code>
 does not refer to
the one declared inside the block.</p>
</li>
<li>
<p>An object's members cannot be made accessible as variables.  The closest
Perl equivalent to <code class="inline"><span class="i">with</span><span class="s">(</span><span class="w">object</span><span class="s">)</span> <span class="s">{</span> <span class="i">method</span><span class="s">(</span><span class="s">)</span> <span class="s">}</span></code>
 is <code class="inline">for</code>
, which can alias
<code class="inline"><span class="i">$_</span></code>
 to the object:</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/for.html">for</a> <span class="s">(</span><span class="i">$object</span><span class="s">)</span> <span class="s">{</span></li><li>	<span class="i">$_</span><span class="i">-&gt;method</span><span class="sc">;</span></li><li>    <span class="s">}</span></li></ol></pre></li>
<li>
<p>The object or class on which a method is called is passed as one of the
method's arguments, not as a separate <code class="inline"><span class="w">this</span></code>
 value.</p>
</li>
</ul>
<a name="Sed-Traps"></a><h2>Sed Traps</h2>
<p>Seasoned <b>sed</b> programmers should take note of the following:</p>
<ul>
<li>
<p>A Perl program executes only once, not once for each input line.  You can
do an implicit loop with <code class="inline">-<span class="w">n</span></code>
 or <code class="inline">-p</code>
.</p>
</li>
<li>
<p>Backreferences in substitutions use "$" rather than "\".</p>
</li>
<li>
<p>The pattern matching metacharacters "(", ")", and "|" do not have backslashes
in front.</p>
</li>
<li>
<p>The range operator is <code class="inline">...</code>
, rather than comma.</p>
</li>
</ul>
<a name="Shell-Traps"></a><h2>Shell Traps</h2>
<p>Sharp shell programmers should take note of the following:</p>
<ul>
<li>
<p>The backtick operator does variable interpolation without regard to
the presence of single quotes in the command.</p>
</li>
<li>
<p>The backtick operator does no translation of the return value, unlike <b>csh</b>.</p>
</li>
<li>
<p>Shells (especially <b>csh</b>) do several levels of substitution on each
command line.  Perl does substitution in only certain constructs
such as double quotes, backticks, angle brackets, and search patterns.</p>
</li>
<li>
<p>Shells interpret scripts a little bit at a time.  Perl compiles the
entire program before executing it (except for <code class="inline">BEGIN</code>
 blocks, which
execute at compile time).</p>
</li>
<li>
<p>The arguments are available via @ARGV, not $1, $2, etc.</p>
</li>
<li>
<p>The environment is not automatically made available as separate scalar
variables.</p>
</li>
<li>
<p>The shell's <code class="inline"><span class="w">test</span></code>
 uses "=", "!=", "&lt;" etc for string comparisons and "-eq",
"-ne", "-lt" etc for numeric comparisons. This is the reverse of Perl, which
uses <code class="inline">eq</code>
, <code class="inline">ne</code>
, <code class="inline">lt</code>
 for string comparisons, and <code class="inline">==</code>
, <code class="inline">!=</code>
 <code class="inline">&lt;</code>
 etc
for numeric comparisons.</p>
</li>
</ul>
<a name="Perl-Traps"></a><h2>Perl Traps</h2>
<p>Practicing Perl Programmers should take note of the following:</p>
<ul>
<li>
<p>Remember that many operations behave differently in a list
context than they do in a scalar one.  See <a href="perldata.html">perldata</a> for details.</p>
</li>
<li>
<p>Avoid barewords if you can, especially all lowercase ones.
You can't tell by just looking at it whether a bareword is
a function or a string.  By using quotes on strings and
parentheses on function calls, you won't ever get them confused.</p>
</li>
<li>
<p>You cannot discern from mere inspection which builtins
are unary operators (like chop() and chdir())
and which are list operators (like print() and unlink()).
(Unless prototyped, user-defined subroutines can <b>only</b> be list
operators, never unary ones.)  See <a href="perlop.html">perlop</a> and <a href="perlsub.html">perlsub</a>.</p>
</li>
<li>
<p>People have a hard time remembering that some functions
default to $_, or @ARGV, or whatever, but that others which
you might expect to do not.</p>
</li>
<li>
<p>The &lt;FH&gt; construct is not the name of the filehandle, it is a readline
operation on that handle.  The data read is assigned to $_ only if the
file read is the sole condition in a while loop:</p>
<pre class="verbatim"><ol><li>    while <span class="s">(</span><span class="q">&lt;FH&gt;</span><span class="s">)</span>      <span class="s">{</span> <span class="s">}</span></li><li>    while <span class="s">(</span><a class="l_k" href="functions/defined.html">defined</a><span class="s">(</span><span class="i">$_</span> = <span class="q">&lt;FH&gt;</span><span class="s">)</span><span class="s">)</span> <span class="s">{</span> <span class="s">}</span>..</li><li>    <span class="q">&lt;FH&gt;</span><span class="sc">;</span>  <span class="c"># data discarded!</span></li></ol></pre></li>
<li>
<p>Remember not to use <code class="inline">=</code>
 when you need <code class="inline">=~</code>
;
these two constructs are quite different:</p>
<pre class="verbatim"><ol><li>    <span class="i">$x</span> =  <span class="q">/foo/</span><span class="sc">;</span></li><li>    <span class="i">$x</span> =~ <span class="q">/foo/</span><span class="sc">;</span></li></ol></pre></li>
<li>
<p>The <code class="inline"><a class="l_k" href="functions/do.html">do</a> <span class="s">{</span><span class="s">}</span></code>
 construct isn't a real loop that you can use
loop control on.</p>
</li>
<li>
<p>Use <code class="inline"><a class="l_k" href="functions/my.html">my()</a></code> for local variables whenever you can get away with
it (but see <a href="perlform.html">perlform</a> for where you can't).
Using <code class="inline"><a class="l_k" href="functions/local.html">local()</a></code> actually gives a local value to a global
variable, which leaves you open to unforeseen side-effects
of dynamic scoping.</p>
</li>
<li>
<p>If you localize an exported variable in a module, its exported value will
not change.  The local name becomes an alias to a new value but the
external name is still an alias for the original.</p>
</li>
</ul>
<p>As always, if any of these are ever officially declared as bugs,
they'll be fixed and removed.</p>




  <div id="page_index" class="hud_container">
    <div id="page_index_header" class="hud_header">
      <div id="page_index_close" class="hud_close"><a href="#" onClick="pageIndex.hide();return false;"></a></div>
      <div id="page_index_title" class="hud_title"><span class="hud_span_top">Page index</span></div>
      <div id="page_index_topright" class="hud_topright"></div>
    </div>
    <div id="page_index_content" class="hud_content">
      <ul><li><a href="#NAME">NAME</a><li><a href="#DESCRIPTION">DESCRIPTION</a><ul><li><a href="#Awk-Traps">Awk Traps</a><li><a href="#C%2fC%2b%2b-Traps">C/C++ Traps</a><li><a href="#JavaScript-Traps">JavaScript Traps</a><li><a href="#Sed-Traps">Sed Traps</a><li><a href="#Shell-Traps">Shell Traps</a><li><a href="#Perl-Traps">Perl Traps</a></ul></ul>
    </div>
    <div id="page_index_footer" class="hud_footer">
      <div id="page_index_bottomleft" class="hud_bottomleft"></div>
      <div id="page_index_bottom" class="hud_bottom"><span class="hud_span_bottom"></span></div>
      <div id="page_index_resize" class="hud_resize"></div>
    </div>
  </div>


	    &nbsp;
          </div>
          <div id="content_footer">
          </div>
        </div>
        <div class="clear"></div>
      </div>
      
    <div id="footer">
      <div id="footer_content">
        <div id="footer_strapline">
          perldoc.perl.org - Official documentation for the Perl programming language
        </div>
        <div id="footer_links">
          <div id="address">
            <p class="name">Contact details</p>
            <p class="address">
	      Site maintained by <a href="mailto:jj@jonallen.info">Jon Allen (JJ)</a><br>
	    </p>
            <p class="contact">
              Documentation maintained by the <a href="http://lists.cpan.org/showlist.cgi?name=perl5-porters">Perl 5 Porters</a>
            </p>
          </div>
          <ul class="f1">
            <li>Manual
              <ul class="f2">
                <li><a href="index-overview.html">Overview</a>
                <li><a href="index-tutorials.html">Tutorials</a>
                <li><a href="index-faq.html">FAQs</a>
                <li><a href="index-history.html">Changes</a>
              </ul>
            <li>Reference
              <ul class="f2">
                <li><a href="index-language.html">Language</a>
                <li><a href="index-functions.html">Functions</a>
                <li><a href="perlop.html">Operators</a>
                <li><a href="perlvar.html">Variables</a>
              </ul>
            <li>Modules
              <ul class="f2">
                <li><a href="index-modules-A.html">Modules</a>
                <li><a href="index-pragmas.html">Pragmas</a>
                <li><a href="index-utilities.html">Utilities</a>
              </ul>
            <li>Misc
              <ul class="f2">
                <li><a href="index-licence.html">License</a>
                <li><a href="index-internals.html">Internals</a>
                <li><a href="index-platforms.html">Platforms</a>
              </ul>          </ul>
          <div class="clear"></div>
        </div>
      </div>
      <div id="footer_end">
      </div>
    </div>
      
    </div>
      <script language="JavaScript" type="text/javascript" src="static/exploreperl.js"></script>
      <script language="JavaScript" src="static/combined-20100403.js" type="text/javascript"></script>
<script language="JavaScript" type="text/javascript">
  perldoc.setPath(0);
  perldoc.pageName    = 'perltrap';
  perldoc.pageAddress = 'perltrap.html';
  perldoc.contentPage = 1;
  explorePerl.render();
  explorePerl.addEvents('explore_anchor');
</script>
    
  </body>
</html>