3 perltrap - Perl traps for the unwary
7 The biggest trap of all is forgetting to use the B<-w> switch; see
8 L<perlrun>. The second biggest trap is not making your entire program
9 runnable under C<use strict>. The third biggest trap is not reading
10 the list of changes in this version of Perl; see L<perldelta>.
14 Accustomed B<awk> users should take special note of the following:
20 The English module, loaded via
24 allows you to refer to special variables (like C<$/>) with names (like
25 C<$RS>), as though they were in B<awk>; see L<perlvar> for details.
29 Semicolons are required after all simple statements in Perl (except
30 at the end of a block). Newline is not a statement delimiter.
34 Curly brackets are required on C<if>s and C<while>s.
38 Variables begin with "$" or "@" in Perl.
42 Arrays index from 0. Likewise string positions in substr() and
47 You have to decide whether your array has numeric or string indices.
51 Hash values do not spring into existence upon mere reference.
55 You have to decide whether you want to use string or numeric
60 Reading an input line does not split it for you. You get to split it
61 to an array yourself. And the split() operator has different
62 arguments than B<awk>'s.
66 The current input line is normally in $_, not $0. It generally does
67 not have the newline stripped. ($0 is the name of the program
68 executed.) See L<perlvar>.
72 $E<lt>I<digit>E<gt> does not refer to fields--it refers to substrings matched
73 by the last match pattern.
77 The print() statement does not add field and record separators unless
78 you set C<$,> and C<$\>. You can set $OFS and $ORS if you're using
83 You must open your files before you print to them.
87 The range operator is "..", not comma. The comma operator works as in
92 The match operator is "=~", not "~". ("~" is the one's complement
97 The exponentiation operator is "**", not "^". "^" is the XOR
98 operator, as in C. (You know, one could get the feeling that B<awk> is
99 basically incompatible with C.)
103 The concatenation operator is ".", not the null string. (Using the
104 null string would render C</pat/ /pat/> unparsable, because the third slash
105 would be interpreted as a division operator--the tokenizer is in fact
106 slightly context sensitive for operators like "/", "?", and "E<gt>".
107 And in fact, "." itself can be the beginning of a number.)
111 The C<next>, C<exit>, and C<continue> keywords work differently.
116 The following variables work differently:
119 ARGC $#ARGV or scalar @ARGV
123 FS (whatever you like)
124 NF $#Fld, or some such
136 You cannot set $RS to a pattern, only a string.
140 When in doubt, run the B<awk> construct through B<a2p> and see what it
147 Cerebral C programmers should take note of the following:
153 Curly brackets are required on C<if>'s and C<while>'s.
157 You must use C<elsif> rather than C<else if>.
161 The C<break> and C<continue> keywords from C become in
162 Perl C<last> and C<next>, respectively.
163 Unlike in C, these do I<NOT> work within a C<do { } while> construct.
167 There's no switch statement. (But it's easy to build one on the fly.)
171 Variables begin with "$" or "@" in Perl.
175 C<printf()> does not implement the "*" format for interpolating
176 field widths, but it's trivial to use interpolation of double-quoted
177 strings to achieve the same effect.
181 Comments begin with "#", not "/*".
185 You can't take the address of anything, although a similar operator
186 in Perl is the backslash, which creates a reference.
190 C<ARGV> must be capitalized. C<$ARGV[0]> is C's C<argv[1]>, and C<argv[0]>
195 System calls such as link(), unlink(), rename(), etc. return nonzero for
200 Signal handlers deal with signal names, not numbers. Use C<kill -l>
201 to find their names on your system.
207 Seasoned B<sed> programmers should take note of the following:
213 Backreferences in substitutions use "$" rather than "\".
217 The pattern matching metacharacters "(", ")", and "|" do not have backslashes
222 The range operator is C<...>, rather than comma.
228 Sharp shell programmers should take note of the following:
234 The backtick operator does variable interpolation without regard to
235 the presence of single quotes in the command.
239 The backtick operator does no translation of the return value, unlike B<csh>.
243 Shells (especially B<csh>) do several levels of substitution on each
244 command line. Perl does substitution in only certain constructs
245 such as double quotes, backticks, angle brackets, and search patterns.
249 Shells interpret scripts a little bit at a time. Perl compiles the
250 entire program before executing it (except for C<BEGIN> blocks, which
251 execute at compile time).
255 The arguments are available via @ARGV, not $1, $2, etc.
259 The environment is not automatically made available as separate scalar
266 Practicing Perl Programmers should take note of the following:
272 Remember that many operations behave differently in a list
273 context than they do in a scalar one. See L<perldata> for details.
277 Avoid barewords if you can, especially all lowercase ones.
278 You can't tell by just looking at it whether a bareword is
279 a function or a string. By using quotes on strings and
280 parentheses on function calls, you won't ever get them confused.
284 You cannot discern from mere inspection which builtins
285 are unary operators (like chop() and chdir())
286 and which are list operators (like print() and unlink()).
287 (User-defined subroutines can be B<only> list operators, never
288 unary ones.) See L<perlop>.
292 People have a hard time remembering that some functions
293 default to $_, or @ARGV, or whatever, but that others which
294 you might expect to do not.
298 The E<lt>FHE<gt> construct is not the name of the filehandle, it is a readline
299 operation on that handle. The data read is assigned to $_ only if the
300 file read is the sole condition in a while loop:
303 while (defined($_ = <FH>)) { }..
304 <FH>; # data discarded!
308 Remember not to use "C<=>" when you need "C<=~>";
309 these two constructs are quite different:
316 The C<do {}> construct isn't a real loop that you can use
321 Use C<my()> for local variables whenever you can get away with
322 it (but see L<perlform> for where you can't).
323 Using C<local()> actually gives a local value to a global
324 variable, which leaves you open to unforeseen side-effects
329 If you localize an exported variable in a module, its exported value will
330 not change. The local name becomes an alias to a new value but the
331 external name is still an alias for the original.
335 =head2 Perl4 to Perl5 Traps
337 Practicing Perl4 Programmers should take note of the following
338 Perl4-to-Perl5 specific traps.
340 They're crudely ordered according to the following list:
344 =item Discontinuance, Deprecation, and BugFix traps
346 Anything that's been fixed as a perl4 bug, removed as a perl4 feature
347 or deprecated as a perl4 feature with the intent to encourage usage of
348 some other perl5 feature.
352 Traps that appear to stem from the new parser.
354 =item Numerical Traps
356 Traps having to do with numerical or mathematical operators.
358 =item General data type traps
360 Traps involving perl standard data types.
362 =item Context Traps - scalar, list contexts
364 Traps related to context within lists, scalar statements/declarations.
366 =item Precedence Traps
368 Traps related to the precedence of parsing, evaluation, and execution of
371 =item General Regular Expression Traps using s///, etc.
373 Traps related to the use of pattern matching.
375 =item Subroutine, Signal, Sorting Traps
377 Traps related to the use of signals and signal handlers, general subroutines,
378 and sorting, along with sorting subroutines.
386 Traps specific to the use of C<dbmopen()>, and specific dbm implementations.
388 =item Unclassified Traps
394 If you find an example of a conversion trap that is not listed here,
395 please submit it to Bill Middleton <F<wjm@best.com>> for inclusion.
396 Also note that at least some of these can be caught with B<-w>.
398 =head2 Discontinuance, Deprecation, and BugFix traps
400 Anything that has been discontinued, deprecated, or fixed as
405 =item * Discontinuance
407 Symbols starting with "_" are no longer forced into package main, except
408 for C<$_> itself (and C<@_>, etc.).
414 print "\$_legacy is ",$_legacy,"\n";
416 # perl4 prints: $_legacy is 1
417 # perl5 prints: $_legacy is
421 Double-colon is now a valid package separator in a variable name. Thus these
422 behave differently in perl4 vs. perl5, because the packages don't exist.
424 $a=1;$b=2;$c=3;$var=4;
426 print "$var::abc::xyz\n";
428 # perl4 prints: 1::2::3 4::abc::xyz
431 Given that C<::> is now the preferred package delimiter, it is debatable
432 whether this should be classed as a bug or not.
433 (The older package delimiter, ' ,is used here)
439 # perl5 prints: Can't find string terminator "'" anywhere before EOF
441 Also see precedence traps, for parsing C<$:>.
445 The second and third arguments of C<splice()> are now evaluated in scalar
446 context (as the Camel says) rather than list context.
448 sub sub1{return(0,2) } # return a 2-elem array
449 sub sub2{ return(1,2,3)} # return a 3-elem array
450 @a1 = ("a","b","c","d","e");
451 @a2 = splice(@a1,&sub1,&sub2);
452 print join(' ',@a2),"\n";
455 # perl5 prints: c d e
457 =item * Discontinuance
459 You can't do a C<goto> into a block that is optimized away. Darn.
465 print "Here I is!\n";
468 # perl4 prints: Here I is!
469 # perl5 dumps core (SEGV)
471 =item * Discontinuance
473 It is no longer syntactically legal to use whitespace as the name
474 of a variable, or as a delimiter for any kind of quote construct.
479 print "a is $a, b is $b\n";
481 # perl4 prints: a is foo bar, b is baz
482 # perl5 errors: Bareword found where operator expected
484 =item * Discontinuance
486 The archaic while/if BLOCK BLOCK syntax is no longer supported.
495 # perl4 prints: True!
496 # perl5 errors: syntax error at test.pl line 1, near "if {"
500 The C<**> operator now binds more tightly than unary minus.
501 It was documented to work this way before, but didn't.
508 =item * Discontinuance
510 The meaning of C<foreach{}> has changed slightly when it is iterating over a
511 list which is not an array. This used to assign the list to a
512 temporary array, but no longer does so (for efficiency). This means
513 that you'll now be iterating over the actual values, not over copies of
514 the values. Modifications to the loop variable can change the original
517 @list = ('ab','abc','bcd','def');
518 foreach $var (grep(/ab/,@list)){
521 print (join(':',@list));
523 # perl4 prints: ab:abc:bcd:def
524 # perl5 prints: 1:1:bcd:def
526 To retain Perl4 semantics you need to assign your list
527 explicitly to a temporary array and then iterate over that. For
528 example, you might need to change
530 foreach $var (grep(/ab/,@list)){
534 foreach $var (@tmp = grep(/ab/,@list)){
536 Otherwise changing $var will clobber the values of @list. (This most often
537 happens when you use C<$_> for the loop variable, and call subroutines in
538 the loop that don't properly localize C<$_>.)
540 =item * Discontinuance
542 C<split> with no arguments now behaves like C<split ' '> (which doesn't
543 return an initial null field if $_ starts with whitespace), it used to
544 behave like C<split /\s+/> (which does).
547 print join(':', split);
549 # perl4 prints: :hi:mom
550 # perl5 prints: hi:mom
554 Perl 4 would ignore any text which was attached to an B<-e> switch,
555 always taking the code snippet from the following arg. Additionally, it
556 would silently accept an B<-e> switch without a following arg. Both of
557 these behaviors have been fixed.
559 perl -e'print "attached to -e"' 'print "separate arg"'
561 # perl4 prints: separate arg
562 # perl5 prints: attached to -e
567 # perl5 dies: No code specified for -e.
569 =item * Discontinuance
571 In Perl 4 the return value of C<push> was undocumented, but it was
572 actually the last value being pushed onto the target list. In Perl 5
573 the return value of C<push> is documented, but has changed, it is the
574 number of elements in the resulting list.
577 print push(@x, 'first new', 'second new');
579 # perl4 prints: second new
582 =item * Discontinuance
584 In Perl 4 (and versions of Perl 5 before 5.004), C<'\r'> characters in
585 Perl code were silently allowed, although they could cause (mysterious!)
586 failures in certain constructs, particularly here documents. Now,
587 C<'\r'> characters cause an immediate fatal error. (Note: In this
588 example, the notation B<\015> represents the incorrect line
589 ending. Depending upon your text viewer, it will look different.)
594 # perl4 prints: foobar
595 # perl5.003 prints: foobar
596 # perl5.004 dies: Illegal character \015 (carriage return)
598 See L<perldiag> for full details.
602 Some error messages will be different.
604 =item * Discontinuance
606 Some bugs may have been inadvertently removed. :-)
612 Perl4-to-Perl5 traps from having to do with parsing.
618 Note the space between . and =
620 $string . = "more string";
623 # perl4 prints: more string
624 # perl5 prints: syntax error at - line 1, near ". ="
628 Better parsing in perl 5
632 print("hello, world\n");
634 # perl4 prints: hello, world
635 # perl5 prints: syntax error
639 "if it looks like a function, it is a function" rule.
642 ($foo == 1) ? "is one\n" : "is zero\n";
644 # perl4 prints: is zero
645 # perl5 warns: "Useless use of a constant in void context" if using -w
649 =head2 Numerical Traps
651 Perl4-to-Perl5 traps having to do with numerical operators,
652 operands, or output from same.
658 Formatted output and significant digits
660 print 7.373504 - 0, "\n";
661 printf "%20.18f\n", 7.373504 - 0;
673 This specific item has been deleted. It demonstrated how the auto-increment
674 operator would not catch when a number went over the signed int limit. Fixed
675 in version 5.003_04. But always be wary when using large integers.
682 Assignment of return values from numeric equality tests
683 does not work in perl5 when the test evaluates to false (0).
684 Logical tests now return an null, instead of 0
692 Also see L<"General Regular Expression Traps using s///, etc.">
693 for another example of this new feature...
697 =head2 General data type traps
699 Perl4-to-Perl5 traps involving most data-types, and their usage
700 within certain expressions and/or context.
706 Negative array subscripts now count from the end of the array.
708 @a = (1, 2, 3, 4, 5);
709 print "The third element of the array is $a[3] also expressed as $a[-2] \n";
711 # perl4 prints: The third element of the array is 4 also expressed as
712 # perl5 prints: The third element of the array is 4 also expressed as 4
716 Setting C<$#array> lower now discards array elements, and makes them
717 impossible to recover.
720 print "Before: ",join('',@a);
722 print ", After: ",join('',@a);
724 print ", Recovered: ",join('',@a),"\n";
726 # perl4 prints: Before: abcde, After: ab, Recovered: abcd
727 # perl5 prints: Before: abcde, After: ab, Recovered: ab
731 Hashes get defined before use
734 die "scalar \$s defined" if defined($s);
735 die "array \@a defined" if defined(@a);
736 die "hash \%h defined" if defined(%h);
739 # perl5 dies: hash %h defined
743 glob assignment from variable to variable will fail if the assigned
744 variable is localized subsequent to the assignment
746 @a = ("This is Perl 4");
751 # perl4 prints: This is Perl 4
756 *fred = *barney; # fred is aliased to barney
759 print "@fred"; # should print "1, 2, 4"
761 # perl4 prints: 1 2 4
762 # perl5 prints: In string, @fred now must be written as \@fred
764 =item * (Scalar String)
766 Changes in unary negation (of strings)
767 This change effects both the return value and what it
768 does to auto(magic)increment.
775 # perl4 prints: aab : -0 : 1
776 # perl5 prints: aab : -aab : aac
780 perl 4 lets you modify constants:
784 for ($x = 0; $x < 3; $x++) {
788 print "before: $_[0]";
790 print " after: $_[0]\n";
801 # Modification of a read-only value attempted at foo.pl line 12.
806 The behavior is slightly different for:
808 print "$x", defined $x
811 # perl 5: <no output, $x is not called into existence>
813 =item * (Variable Suicide)
815 Variable suicide behavior is more consistent under Perl 5.
816 Perl5 exhibits the same behavior for hashes and scalars,
817 that perl4 exhibits for only scalars.
819 $aGlobal{ "aKey" } = "global value";
820 print "MAIN:", $aGlobal{"aKey"}, "\n";
825 local( *theArgument ) = @_;
826 local( %aNewLocal ); # perl 4 != 5.001l,m
827 $aNewLocal{"aKey"} = "this should never appear";
828 print "SUB: ", $theArgument{"aKey"}, "\n";
829 $aNewLocal{"aKey"} = "level $GlobalLevel"; # what should print
831 if( $GlobalLevel<4 ) {
846 # SUB: this should never appear
847 # SUB: this should never appear
848 # SUB: this should never appear
852 =head2 Context Traps - scalar, list contexts
856 =item * (list context)
858 The elements of argument lists for formats are now evaluated in list
859 context. This means you can interpolate list values now.
861 @fmt = ("foo","bar","baz");
868 # perl4 errors: Please use commas to separate fields in file
869 # perl5 prints: foo bar baz
871 =item * (scalar context)
873 The C<caller()> function now returns a false value in a scalar context
874 if there is no caller. This lets library files determine if they're
877 caller() ? (print "You rang?\n") : (print "Got a 0\n");
879 # perl4 errors: There is no caller
880 # perl5 prints: Got a 0
882 =item * (scalar context)
884 The comma operator in a scalar context is now guaranteed to give a
885 scalar context to its arguments.
891 # Perl4 prints: x = c # Thinks list context interpolates list
892 # Perl5 prints: x = 3 # Knows scalar uses length of list
894 =item * (list, builtin)
896 C<sprintf()> funkiness (array argument converted to scalar array count)
897 This test could be added to t/op/sprintf.t
899 @z = ('%s%s', 'foo', 'bar');
901 if ($x eq 'foobar') {print "ok 2\n";} else {print "not ok 2 '$x'\n";}
904 # perl5 prints: not ok 2
906 C<printf()> works fine, though:
911 # perl4 prints: foobar
912 # perl5 prints: foobar
918 =head2 Precedence Traps
920 Perl4-to-Perl5 traps involving precedence order.
926 LHS vs. RHS when both sides are getting an op.
928 @arr = ( 'left', 'right' );
929 $a{shift @arr} = shift @arr;
930 print join( ' ', keys %a );
933 # perl5 prints: right
937 These are now semantic errors because of precedence:
940 %map = ("a",1,"b",2,"c",3,"d",4);
941 $n = shift @list + 2; # first item in list plus 2
943 $m = keys %map + 2; # number of items in hash plus 2
946 # perl4 prints: n is 3, m is 6
947 # perl5 errors and fails to compile
951 The precedence of assignment operators is now the same as the precedence
952 of assignment. Perl 4 mistakenly gave them the precedence of the associated
953 operator. So you now must parenthesize them in expressions like
955 /foo/ ? ($a += 2) : ($a -= 2);
959 /foo/ ? $a += 2 : $a -= 2
961 would be erroneously parsed as
963 (/foo/ ? $a += 2 : $a) -= 2;
969 now works as a C programmer would expect.
975 is now incorrect. You need parentheses around the filehandle.
976 Otherwise, perl5 leaves the statement as its default precedence:
980 # perl4 opens or dies
981 # perl5 errors: Precedence problem: open FOO should be open(FOO)
985 perl4 gives the special variable, C<$:> precedence, where perl5
986 treats C<$::> as main C<package>
988 $a = "x"; print "$::a";
995 concatenation precedence over filetest operator?
999 # perl4 prints: no output
1000 # perl5 prints: Can't modify -e in concatenation
1004 Assignment to value takes precedence over assignment to key in
1005 perl5 when using the shift operator on both sides.
1007 @arr = ( 'left', 'right' );
1008 $a{shift @arr} = shift @arr;
1009 print join( ' ', keys %a );
1011 # perl4 prints: left
1012 # perl5 prints: right
1016 =head2 General Regular Expression Traps using s///, etc.
1018 All types of RE traps.
1022 =item * Regular Expression
1024 C<s'$lhs'$rhs'> now does no interpolation on either side. It used to
1025 interpolate C<$lhs> but not C<$rhs>. (And still does not match a literal
1029 $string = '1 2 $a $b';
1030 $string =~ s'$a'$b';
1033 # perl4 prints: $b 2 $a $b
1034 # perl5 prints: 1 2 $a $b
1036 =item * Regular Expression
1038 C<m//g> now attaches its state to the searched string rather than the
1039 regular expression. (Once the scope of a block is left for the sub, the
1040 state of the searched string is lost)
1046 sub doit{local($_) = shift; print "Got $_ "}
1048 # perl4 prints: blah blah blah
1049 # perl5 prints: infinite loop blah...
1051 =item * Regular Expression
1053 Currently, if you use the C<m//o> qualifier on a regular expression
1054 within an anonymous sub, I<all> closures generated from that anonymous
1055 sub will use the regular expression as it was compiled when it was used
1056 the very first time in any such closure. For instance, if you say
1059 my($left,$right) = @_;
1060 return sub { $_[0] =~ /$left stuff $right/o; };
1063 build_match() will always return a sub which matches the contents of
1064 C<$left> and C<$right> as they were the I<first> time that build_match()
1065 was called, not as they are in the current call.
1067 This is probably a bug, and may change in future versions of Perl.
1069 =item * Regular Expression
1071 If no parentheses are used in a match, Perl4 sets C<$+> to
1072 the whole match, just like C<$&>. Perl5 does not.
1077 # perl4 prints: bcde
1080 =item * Regular Expression
1082 substitution now returns the null string if it fails
1085 $value = ($string =~ s/foo//);
1091 Also see L<Numerical Traps> for another example of this new feature.
1093 =item * Regular Expression
1095 C<s`lhs`rhs`> (using backticks) is now a normal substitution, with no
1099 $string =~ s`^`hostname`;
1100 print $string, "\n";
1102 # perl4 prints: <the local hostname>
1103 # perl5 prints: hostname
1105 =item * Regular Expression
1107 Stricter parsing of variables used in regular expressions
1109 s/^([^$grpc]*$grpc[$opt$plus$rep]?)//o;
1111 # perl4: compiles w/o error
1112 # perl5: with Scalar found where operator expected ..., near "$opt$plus"
1114 an added component of this example, apparently from the same script, is
1115 the actual value of the s'd string after the substitution.
1116 C<[$opt]> is a character class in perl4 and an array subscript in perl5
1121 s/^([^$grpc]*$grpc[$opt]?)/foo/;
1125 # perl5 prints: foobar
1127 =item * Regular Expression
1129 Under perl5, C<m?x?> matches only once, like C<?x?>. Under perl4, it matched
1130 repeatedly, like C</x/> or C<m!x!>.
1133 sub match { $test =~ m?once?; }
1136 # m?x? matches more then once
1139 # m?x? matches only once
1143 # perl4 prints: perl4
1144 # perl5 prints: perl5
1147 =item * Regular Expression
1149 Under perl4 and upto version 5.003, a failed C<m//g> match used to
1150 reset the internal iterator, so that subsequent C<m//g> match attempts
1151 began from the beginning of the string. In perl version 5.004 and later,
1152 failed C<m//g> matches do not reset the iterator position (which can be
1153 found using the C<pos()> function--see L<perlfunc/pos>).
1157 print $1 while ($test =~ /(o)/g);
1158 # pos $test = 0; # to get old behavior
1161 # perl4 prints: oooooo
1162 # perl5.004 prints: oo
1164 You may always reset the iterator yourself as shown in the commented line
1165 to get the old behavior.
1169 =head2 Subroutine, Signal, Sorting Traps
1171 The general group of Perl4-to-Perl5 traps having to do with
1172 Signals, Sorting, and their related subroutines, as well as
1173 general subroutine traps. Includes some OS-Specific traps.
1179 Barewords that used to look like strings to Perl will now look like subroutine
1180 calls if a subroutine by that name is defined before the compiler sees them.
1182 sub SeeYa { warn"Hasta la vista, baby!" }
1183 $SIG{'TERM'} = SeeYa;
1184 print "SIGTERM is now $SIG{'TERM'}\n";
1186 # perl4 prints: SIGTERM is main'SeeYa
1187 # perl5 prints: SIGTERM is now main::1
1189 Use B<-w> to catch this one
1191 =item * (Sort Subroutine)
1193 reverse is no longer allowed as the name of a sort subroutine.
1195 sub reverse{ print "yup "; $a <=> $b }
1196 print sort reverse a,b,c;
1198 # perl4 prints: yup yup yup yup abc
1201 =item * warn() won't let you specify a filehandle.
1203 Although it _always_ printed to STDERR, warn() would let you specify a
1204 filehandle in perl4. With perl5 it does not.
1208 # perl4 prints: Foo!
1209 # perl5 prints: String found where operator expected
1219 Under HPUX, and some other SysV OSes, one had to reset any signal handler,
1220 within the signal handler function, each time a signal was handled with
1221 perl4. With perl5, the reset is now done correctly. Any code relying
1222 on the handler _not_ being reset will have to be reworked.
1224 Since version 5.002, Perl uses sigaction() under SysV.
1229 $SIG{'INT'} = 'gotit';
1238 while (1) {sleep(10);}
1241 # perl4 (HPUX) prints: Got INT...
1242 # perl5 (HPUX) prints: Got INT... Got INT...
1246 Under SysV OSes, C<seek()> on a file opened to append C<E<gt>E<gt>> now does
1247 the right thing w.r.t. the fopen() manpage. e.g., - When a file is opened
1248 for append, it is impossible to overwrite information already in
1251 open(TEST,">>seek.test");
1252 $start = tell TEST ;
1257 seek(TEST,$start,0);
1258 print TEST "18 characters here";
1260 # perl4 (solaris) seek.test has: 18 characters here
1261 # perl5 (solaris) seek.test has: 1 2 3 4 5 6 7 8 9 18 characters here
1267 =head2 Interpolation Traps
1269 Perl4-to-Perl5 traps having to do with how things get interpolated
1270 within certain expressions, statements, contexts, or whatever.
1274 =item * Interpolation
1276 @ now always interpolates an array in double-quotish strings.
1278 print "To: someone@somewhere.com\n";
1280 # perl4 prints: To:someone@somewhere.com
1281 # perl5 errors : In string, @somewhere now must be written as \@somewhere
1283 =item * Interpolation
1285 Double-quoted strings may no longer end with an unescaped $ or @.
1289 print "foo is $foo, bar is $bar\n";
1291 # perl4 prints: foo is foo$, bar is bar@
1292 # perl5 errors: Final $ should be \$ or $name
1294 Note: perl5 DOES NOT error on the terminating @ in $bar
1296 =item * Interpolation
1298 Perl now sometimes evaluates arbitrary expressions inside braces that occur
1299 within double quotes (usually when the opening brace is preceded by C<$>
1305 sub foo { return "bar" };
1306 print "|@{w.w.w}|${main'foo}|";
1308 # perl4 prints: |@{w.w.w}|foo|
1309 # perl5 prints: |buz|bar|
1311 Note that you can C<use strict;> to ward off such trappiness under perl5.
1313 =item * Interpolation
1315 The construct "this is $$x" used to interpolate the pid at that
1316 point, but now apparently tries to dereference C<$x>. C<$$> by itself still
1317 works fine, however.
1319 print "this is $$x\n";
1321 # perl4 prints: this is XXXx (XXX is the current pid)
1322 # perl5 prints: this is
1324 =item * Interpolation
1326 Creation of hashes on the fly with C<eval "EXPR"> now requires either both
1327 C<$>'s to be protected in the specification of the hash name, or both curlies
1328 to be protected. If both curlies are protected, the result will be compatible
1329 with perl4 and perl5. This is a very common practice, and should be changed
1330 to use the block form of C<eval{}> if possible.
1332 $hashname = "foobar";
1335 eval "\$$hashname{'$key'} = q|$value|";
1336 (defined($foobar{'baz'})) ? (print "Yup") : (print "Nope");
1339 # perl5 prints: Nope
1343 eval "\$$hashname{'$key'} = q|$value|";
1347 eval "\$\$hashname{'$key'} = q|$value|";
1349 causes the following result:
1351 # perl4 prints: Nope
1356 eval "\$$hashname\{'$key'\} = q|$value|";
1358 causes the following result:
1362 # and is compatible for both versions
1365 =item * Interpolation
1367 perl4 programs which unconsciously rely on the bugs in earlier perl versions.
1369 perl -e '$bar=q/not/; print "This is $foo{$bar} perl5"'
1371 # perl4 prints: This is not perl5
1372 # perl5 prints: This is perl5
1374 =item * Interpolation
1376 You also have to be careful about array references.
1381 perl 5 prints: syntax error
1383 =item * Interpolation
1385 Similarly, watch out for:
1388 print "\$$foo{bar}\n";
1390 # perl4 prints: $array{bar}
1393 Perl 5 is looking for C<$array{bar}> which doesn't exist, but perl 4 is
1394 happy just to expand $foo to "array" by itself. Watch out for this
1395 especially in C<eval>'s.
1397 =item * Interpolation
1399 C<qq()> string passed to C<eval>
1402 foreach \$y (keys %\$x\) {
1407 # perl4 runs this ok
1408 # perl5 prints: Can't find string terminator ")"
1420 Existing dbm databases created under perl4 (or any other dbm/ndbm tool)
1421 may cause the same script, run under perl5, to fail. The build of perl5
1422 must have been linked with the same dbm/ndbm as the default for C<dbmopen()>
1423 to function properly without C<tie>'ing to an extension dbm implementation.
1425 dbmopen (%dbm, "file", undef);
1429 # perl5 prints: ok (IFF linked with -ldbm or -lndbm)
1434 Existing dbm databases created under perl4 (or any other dbm/ndbm tool)
1435 may cause the same script, run under perl5, to fail. The error generated
1436 when exceeding the limit on the key/value size will cause perl5 to exit
1439 dbmopen(DB, "testdb",0600) || die "couldn't open db! $!";
1440 $DB{'trap'} = "x" x 1024; # value too large for most dbm/ndbm
1444 dbm store returned -1, errno 28, key "trap" at - line 3.
1448 dbm store returned -1, errno 28, key "trap" at - line 3.
1452 =head2 Unclassified Traps
1458 =item * Unclassified
1460 C<require>/C<do> trap using returned value
1462 If the file doit.pl has:
1470 And the do.pl file has the following single line:
1474 Running doit.pl gives the following:
1476 # perl 4 prints: 3 (aborts the subroutine early)
1479 Same behavior if you replace C<do> with C<require>.
1483 As always, if any of these are ever officially declared as bugs,
1484 they'll be fixed and removed.