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 $RS), as though they were in B<awk>; see L<perlvar> for details.
29 Semicolons are required after all simple statements in Perl (except
30 at the end of a block). Newline is not a statement delimiter.
34 Curly brackets are required on C<if>s and C<while>s.
38 Variables begin with "$", "@" 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 $<I<digit>> does not refer to fields--it refers to substrings matched
73 by the last match pattern.
77 The print() statement does not add field and record separators unless
78 you set C<$,> 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 ">".
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 Comments begin with "#", not "/*".
179 You can't take the address of anything, although a similar operator
180 in Perl is the backslash, which creates a reference.
184 C<ARGV> must be capitalized. C<$ARGV[0]> is C's C<argv[1]>, and C<argv[0]>
189 System calls such as link(), unlink(), rename(), etc. return nonzero for
194 Signal handlers deal with signal names, not numbers. Use C<kill -l>
195 to find their names on your system.
201 Seasoned B<sed> programmers should take note of the following:
207 Backreferences in substitutions use "$" rather than "\".
211 The pattern matching metacharacters "(", ")", and "|" do not have backslashes
216 The range operator is C<...>, rather than comma.
222 Sharp shell programmers should take note of the following:
228 The backtick operator does variable interpolation without regard to
229 the presence of single quotes in the command.
233 The backtick operator does no translation of the return value, unlike B<csh>.
237 Shells (especially B<csh>) do several levels of substitution on each
238 command line. Perl does substitution in only certain constructs
239 such as double quotes, backticks, angle brackets, and search patterns.
243 Shells interpret scripts a little bit at a time. Perl compiles the
244 entire program before executing it (except for C<BEGIN> blocks, which
245 execute at compile time).
249 The arguments are available via @ARGV, not $1, $2, etc.
253 The environment is not automatically made available as separate scalar
260 Practicing Perl Programmers should take note of the following:
266 Remember that many operations behave differently in a list
267 context than they do in a scalar one. See L<perldata> for details.
271 Avoid barewords if you can, especially all lowercase ones.
272 You can't tell by just looking at it whether a bareword is
273 a function or a string. By using quotes on strings and
274 parentheses on function calls, you won't ever get them confused.
278 You cannot discern from mere inspection which builtins
279 are unary operators (like chop() and chdir())
280 and which are list operators (like print() and unlink()).
281 (User-defined subroutines can be B<only> list operators, never
282 unary ones.) See L<perlop>.
286 People have a hard time remembering that some functions
287 default to $_, or @ARGV, or whatever, but that others which
288 you might expect to do not.
292 The <FH> construct is not the name of the filehandle, it is a readline
293 operation on that handle. The data read is assigned to $_ only if the
294 file read is the sole condition in a while loop:
297 while (defined($_ = <FH>)) { }..
298 <FH>; # data discarded!
302 Remember not to use C<=> when you need C<=~>;
303 these two constructs are quite different:
310 The C<do {}> construct isn't a real loop that you can use
315 Use C<my()> for local variables whenever you can get away with
316 it (but see L<perlform> for where you can't).
317 Using C<local()> actually gives a local value to a global
318 variable, which leaves you open to unforeseen side-effects
323 If you localize an exported variable in a module, its exported value will
324 not change. The local name becomes an alias to a new value but the
325 external name is still an alias for the original.
329 =head2 Perl4 to Perl5 Traps
331 Practicing Perl4 Programmers should take note of the following
332 Perl4-to-Perl5 specific traps.
334 They're crudely ordered according to the following list:
338 =item Discontinuance, Deprecation, and BugFix traps
340 Anything that's been fixed as a perl4 bug, removed as a perl4 feature
341 or deprecated as a perl4 feature with the intent to encourage usage of
342 some other perl5 feature.
346 Traps that appear to stem from the new parser.
348 =item Numerical Traps
350 Traps having to do with numerical or mathematical operators.
352 =item General data type traps
354 Traps involving perl standard data types.
356 =item Context Traps - scalar, list contexts
358 Traps related to context within lists, scalar statements/declarations.
360 =item Precedence Traps
362 Traps related to the precedence of parsing, evaluation, and execution of
365 =item General Regular Expression Traps using s///, etc.
367 Traps related to the use of pattern matching.
369 =item Subroutine, Signal, Sorting Traps
371 Traps related to the use of signals and signal handlers, general subroutines,
372 and sorting, along with sorting subroutines.
380 Traps specific to the use of C<dbmopen()>, and specific dbm implementations.
382 =item Unclassified Traps
388 If you find an example of a conversion trap that is not listed here,
389 please submit it to <F<perlbug@perl.org>> for inclusion.
390 Also note that at least some of these can be caught with the
391 C<use warnings> pragma or the B<-w> switch.
393 =head2 Discontinuance, Deprecation, and BugFix traps
395 Anything that has been discontinued, deprecated, or fixed as
400 =item * Discontinuance
402 Symbols starting with "_" are no longer forced into package main, except
403 for C<$_> itself (and C<@_>, etc.).
409 print "\$_legacy is ",$_legacy,"\n";
411 # perl4 prints: $_legacy is 1
412 # perl5 prints: $_legacy is
416 Double-colon is now a valid package separator in a variable name. Thus these
417 behave differently in perl4 vs. perl5, because the packages don't exist.
419 $a=1;$b=2;$c=3;$var=4;
421 print "$var::abc::xyz\n";
423 # perl4 prints: 1::2::3 4::abc::xyz
426 Given that C<::> is now the preferred package delimiter, it is debatable
427 whether this should be classed as a bug or not.
428 (The older package delimiter, ' ,is used here)
434 # perl5 prints: Can't find string terminator "'" anywhere before EOF
436 You can avoid this problem, and remain compatible with perl4, if you
437 always explicitly include the package name:
440 print "x=${main'x}\n" ;
442 Also see precedence traps, for parsing C<$:>.
446 The second and third arguments of C<splice()> are now evaluated in scalar
447 context (as the Camel says) rather than list context.
449 sub sub1{return(0,2) } # return a 2-element list
450 sub sub2{ return(1,2,3)} # return a 3-element list
451 @a1 = ("a","b","c","d","e");
452 @a2 = splice(@a1,&sub1,&sub2);
453 print join(' ',@a2),"\n";
456 # perl5 prints: c d e
458 =item * Discontinuance
460 You can't do a C<goto> into a block that is optimized away. Darn.
466 print "Here I is!\n";
469 # perl4 prints: Here I is!
470 # perl5 dumps core (SEGV)
472 =item * Discontinuance
474 It is no longer syntactically legal to use whitespace as the name
475 of a variable, or as a delimiter for any kind of quote construct.
480 print "a is $a, b is $b\n";
482 # perl4 prints: a is foo bar, b is baz
483 # perl5 errors: Bareword found where operator expected
485 =item * Discontinuance
487 The archaic while/if BLOCK BLOCK syntax is no longer supported.
496 # perl4 prints: True!
497 # perl5 errors: syntax error at test.pl line 1, near "if {"
501 The C<**> operator now binds more tightly than unary minus.
502 It was documented to work this way before, but didn't.
509 =item * Discontinuance
511 The meaning of C<foreach{}> has changed slightly when it is iterating over a
512 list which is not an array. This used to assign the list to a
513 temporary array, but no longer does so (for efficiency). This means
514 that you'll now be iterating over the actual values, not over copies of
515 the values. Modifications to the loop variable can change the original
518 @list = ('ab','abc','bcd','def');
519 foreach $var (grep(/ab/,@list)){
522 print (join(':',@list));
524 # perl4 prints: ab:abc:bcd:def
525 # perl5 prints: 1:1:bcd:def
527 To retain Perl4 semantics you need to assign your list
528 explicitly to a temporary array and then iterate over that. For
529 example, you might need to change
531 foreach $var (grep(/ab/,@list)){
535 foreach $var (@tmp = grep(/ab/,@list)){
537 Otherwise changing $var will clobber the values of @list. (This most often
538 happens when you use C<$_> for the loop variable, and call subroutines in
539 the loop that don't properly localize C<$_>.)
541 =item * Discontinuance
543 C<split> with no arguments now behaves like C<split ' '> (which doesn't
544 return an initial null field if $_ starts with whitespace), it used to
545 behave like C<split /\s+/> (which does).
548 print join(':', split);
550 # perl4 prints: :hi:mom
551 # perl5 prints: hi:mom
555 Perl 4 would ignore any text which was attached to an B<-e> switch,
556 always taking the code snippet from the following arg. Additionally, it
557 would silently accept an B<-e> switch without a following arg. Both of
558 these behaviors have been fixed.
560 perl -e'print "attached to -e"' 'print "separate arg"'
562 # perl4 prints: separate arg
563 # perl5 prints: attached to -e
568 # perl5 dies: No code specified for -e.
570 =item * Discontinuance
572 In Perl 4 the return value of C<push> was undocumented, but it was
573 actually the last value being pushed onto the target list. In Perl 5
574 the return value of C<push> is documented, but has changed, it is the
575 number of elements in the resulting list.
578 print push(@x, 'first new', 'second new');
580 # perl4 prints: second new
585 Some error messages will be different.
587 =item * Discontinuance
589 In Perl 4, if in list context the delimiters to the first argument of
590 C<split()> were C<??>, the result would be placed in C<@_> as well as
591 being returned. Perl 5 has more respect for your subroutine arguments.
593 =item * Discontinuance
595 Some bugs may have been inadvertently removed. :-)
601 Perl4-to-Perl5 traps from having to do with parsing.
607 Note the space between . and =
609 $string . = "more string";
612 # perl4 prints: more string
613 # perl5 prints: syntax error at - line 1, near ". ="
617 Better parsing in perl 5
621 print("hello, world\n");
623 # perl4 prints: hello, world
624 # perl5 prints: syntax error
628 "if it looks like a function, it is a function" rule.
631 ($foo == 1) ? "is one\n" : "is zero\n";
633 # perl4 prints: is zero
634 # perl5 warns: "Useless use of a constant in void context" if using -w
638 String interpolation of the C<$#array> construct differs when braces
639 are to used around the name.
645 # perl5 fails with syntax error
655 =head2 Numerical Traps
657 Perl4-to-Perl5 traps having to do with numerical operators,
658 operands, or output from same.
664 Formatted output and significant digits
666 print 7.373504 - 0, "\n";
667 printf "%20.18f\n", 7.373504 - 0;
679 This specific item has been deleted. It demonstrated how the auto-increment
680 operator would not catch when a number went over the signed int limit. Fixed
681 in version 5.003_04. But always be wary when using large integers.
688 Assignment of return values from numeric equality tests
689 does not work in perl5 when the test evaluates to false (0).
690 Logical tests now return an null, instead of 0
698 Also see L<"General Regular Expression Traps using s///, etc.">
699 for another example of this new feature...
701 =item * Bitwise string ops
703 When bitwise operators which can operate upon either numbers or
704 strings (C<& | ^ ~>) are given only strings as arguments, perl4 would
705 treat the operands as bitstrings so long as the program contained a call
706 to the C<vec()> function. perl5 treats the string operands as bitstrings.
707 (See L<perlop/Bitwise String Operators> for more details.)
711 $betty = $fred & $barney;
713 # Uncomment the next line to change perl4's behavior
714 # ($dummy) = vec("dummy", 0, 0);
722 # If vec() is used anywhere in the program, both print:
727 =head2 General data type traps
729 Perl4-to-Perl5 traps involving most data-types, and their usage
730 within certain expressions and/or context.
736 Negative array subscripts now count from the end of the array.
738 @a = (1, 2, 3, 4, 5);
739 print "The third element of the array is $a[3] also expressed as $a[-2] \n";
741 # perl4 prints: The third element of the array is 4 also expressed as
742 # perl5 prints: The third element of the array is 4 also expressed as 4
746 Setting C<$#array> lower now discards array elements, and makes them
747 impossible to recover.
750 print "Before: ",join('',@a);
752 print ", After: ",join('',@a);
754 print ", Recovered: ",join('',@a),"\n";
756 # perl4 prints: Before: abcde, After: ab, Recovered: abcd
757 # perl5 prints: Before: abcde, After: ab, Recovered: ab
761 Hashes get defined before use
764 die "scalar \$s defined" if defined($s);
765 die "array \@a defined" if defined(@a);
766 die "hash \%h defined" if defined(%h);
769 # perl5 dies: hash %h defined
771 Perl will now generate a warning when it sees defined(@a) and
776 glob assignment from variable to variable will fail if the assigned
777 variable is localized subsequent to the assignment
779 @a = ("This is Perl 4");
784 # perl4 prints: This is Perl 4
789 Assigning C<undef> to a glob has no effect in Perl 5. In Perl 4
790 it undefines the associated scalar (but may have other side effects
793 =item * (Scalar String)
795 Changes in unary negation (of strings)
796 This change effects both the return value and what it
797 does to auto(magic)increment.
804 # perl4 prints: aab : -0 : 1
805 # perl5 prints: aab : -aab : aac
809 perl 4 lets you modify constants:
813 for ($x = 0; $x < 3; $x++) {
817 print "before: $_[0]";
819 print " after: $_[0]\n";
830 # Modification of a read-only value attempted at foo.pl line 12.
835 The behavior is slightly different for:
837 print "$x", defined $x
840 # perl 5: <no output, $x is not called into existence>
842 =item * (Variable Suicide)
844 Variable suicide behavior is more consistent under Perl 5.
845 Perl5 exhibits the same behavior for hashes and scalars,
846 that perl4 exhibits for only scalars.
848 $aGlobal{ "aKey" } = "global value";
849 print "MAIN:", $aGlobal{"aKey"}, "\n";
854 local( *theArgument ) = @_;
855 local( %aNewLocal ); # perl 4 != 5.001l,m
856 $aNewLocal{"aKey"} = "this should never appear";
857 print "SUB: ", $theArgument{"aKey"}, "\n";
858 $aNewLocal{"aKey"} = "level $GlobalLevel"; # what should print
860 if( $GlobalLevel<4 ) {
875 # SUB: this should never appear
876 # SUB: this should never appear
877 # SUB: this should never appear
881 =head2 Context Traps - scalar, list contexts
885 =item * (list context)
887 The elements of argument lists for formats are now evaluated in list
888 context. This means you can interpolate list values now.
890 @fmt = ("foo","bar","baz");
897 # perl4 errors: Please use commas to separate fields in file
898 # perl5 prints: foo bar baz
900 =item * (scalar context)
902 The C<caller()> function now returns a false value in a scalar context
903 if there is no caller. This lets library files determine if they're
906 caller() ? (print "You rang?\n") : (print "Got a 0\n");
908 # perl4 errors: There is no caller
909 # perl5 prints: Got a 0
911 =item * (scalar context)
913 The comma operator in a scalar context is now guaranteed to give a
914 scalar context to its arguments.
920 # Perl4 prints: x = c # Thinks list context interpolates list
921 # Perl5 prints: x = 3 # Knows scalar uses length of list
923 =item * (list, builtin)
925 C<sprintf()> funkiness (array argument converted to scalar array count)
926 This test could be added to t/op/sprintf.t
928 @z = ('%s%s', 'foo', 'bar');
930 if ($x eq 'foobar') {print "ok 2\n";} else {print "not ok 2 '$x'\n";}
933 # perl5 prints: not ok 2
935 C<printf()> works fine, though:
940 # perl4 prints: foobar
941 # perl5 prints: foobar
947 =head2 Precedence Traps
949 Perl4-to-Perl5 traps involving precedence order.
951 Perl 4 has almost the same precedence rules as Perl 5 for the operators
952 that they both have. Perl 4 however, seems to have had some
953 inconsistencies that made the behavior differ from what was documented.
959 LHS vs. RHS of any assignment operator. LHS is evaluated first
960 in perl4, second in perl5; this can affect the relationship
961 between side-effects in sub-expressions.
963 @arr = ( 'left', 'right' );
964 $a{shift @arr} = shift @arr;
965 print join( ' ', keys %a );
968 # perl5 prints: right
972 These are now semantic errors because of precedence:
975 %map = ("a",1,"b",2,"c",3,"d",4);
976 $n = shift @list + 2; # first item in list plus 2
978 $m = keys %map + 2; # number of items in hash plus 2
981 # perl4 prints: n is 3, m is 6
982 # perl5 errors and fails to compile
986 The precedence of assignment operators is now the same as the precedence
987 of assignment. Perl 4 mistakenly gave them the precedence of the associated
988 operator. So you now must parenthesize them in expressions like
990 /foo/ ? ($a += 2) : ($a -= 2);
994 /foo/ ? $a += 2 : $a -= 2
996 would be erroneously parsed as
998 (/foo/ ? $a += 2 : $a) -= 2;
1002 $a += /foo/ ? 1 : 2;
1004 now works as a C programmer would expect.
1010 is now incorrect. You need parentheses around the filehandle.
1011 Otherwise, perl5 leaves the statement as its default precedence:
1015 # perl4 opens or dies
1016 # perl5 errors: Precedence problem: open FOO should be open(FOO)
1020 perl4 gives the special variable, C<$:> precedence, where perl5
1021 treats C<$::> as main C<package>
1023 $a = "x"; print "$::a";
1025 # perl 4 prints: -:a
1030 perl4 had buggy precedence for the file test operators vis-a-vis
1031 the assignment operators. Thus, although the precedence table
1032 for perl4 leads one to believe C<-e $foo .= "q"> should parse as
1033 C<((-e $foo) .= "q")>, it actually parses as C<(-e ($foo .= "q"))>.
1034 In perl5, the precedence is as documented.
1038 # perl4 prints: no output
1039 # perl5 prints: Can't modify -e in concatenation
1043 In perl4, keys(), each() and values() were special high-precedence operators
1044 that operated on a single hash, but in perl5, they are regular named unary
1045 operators. As documented, named unary operators have lower precedence
1046 than the arithmetic and concatenation operators C<+ - .>, but the perl4
1047 variants of these operators actually bind tighter than C<+ - .>.
1054 # perl5 prints: Type of arg 1 to keys must be hash (not subtraction)
1056 The perl4 behavior was probably more useful, if less consistent.
1060 =head2 General Regular Expression Traps using s///, etc.
1062 All types of RE traps.
1066 =item * Regular Expression
1068 C<s'$lhs'$rhs'> now does no interpolation on either side. It used to
1069 interpolate $lhs but not $rhs. (And still does not match a literal
1073 $string = '1 2 $a $b';
1074 $string =~ s'$a'$b';
1077 # perl4 prints: $b 2 $a $b
1078 # perl5 prints: 1 2 $a $b
1080 =item * Regular Expression
1082 C<m//g> now attaches its state to the searched string rather than the
1083 regular expression. (Once the scope of a block is left for the sub, the
1084 state of the searched string is lost)
1090 sub doit{local($_) = shift; print "Got $_ "}
1092 # perl4 prints: blah blah blah
1093 # perl5 prints: infinite loop blah...
1095 =item * Regular Expression
1097 Currently, if you use the C<m//o> qualifier on a regular expression
1098 within an anonymous sub, I<all> closures generated from that anonymous
1099 sub will use the regular expression as it was compiled when it was used
1100 the very first time in any such closure. For instance, if you say
1103 my($left,$right) = @_;
1104 return sub { $_[0] =~ /$left stuff $right/o; };
1107 build_match() will always return a sub which matches the contents of
1108 $left and $right as they were the I<first> time that build_match()
1109 was called, not as they are in the current call.
1111 This is probably a bug, and may change in future versions of Perl.
1113 =item * Regular Expression
1115 If no parentheses are used in a match, Perl4 sets C<$+> to
1116 the whole match, just like C<$&>. Perl5 does not.
1121 # perl4 prints: bcde
1124 =item * Regular Expression
1126 substitution now returns the null string if it fails
1129 $value = ($string =~ s/foo//);
1135 Also see L<Numerical Traps> for another example of this new feature.
1137 =item * Regular Expression
1139 C<s`lhs`rhs`> (using backticks) is now a normal substitution, with no
1143 $string =~ s`^`hostname`;
1144 print $string, "\n";
1146 # perl4 prints: <the local hostname>
1147 # perl5 prints: hostname
1149 =item * Regular Expression
1151 Stricter parsing of variables used in regular expressions
1153 s/^([^$grpc]*$grpc[$opt$plus$rep]?)//o;
1155 # perl4: compiles w/o error
1156 # perl5: with Scalar found where operator expected ..., near "$opt$plus"
1158 an added component of this example, apparently from the same script, is
1159 the actual value of the s'd string after the substitution.
1160 C<[$opt]> is a character class in perl4 and an array subscript in perl5
1165 s/^([^$grpc]*$grpc[$opt]?)/foo/;
1169 # perl5 prints: foobar
1171 =item * Regular Expression
1173 Under perl5, C<m?x?> matches only once, like C<?x?>. Under perl4, it matched
1174 repeatedly, like C</x/> or C<m!x!>.
1177 sub match { $test =~ m?once?; }
1180 # m?x? matches more then once
1183 # m?x? matches only once
1187 # perl4 prints: perl4
1188 # perl5 prints: perl5
1193 =head2 Subroutine, Signal, Sorting Traps
1195 The general group of Perl4-to-Perl5 traps having to do with
1196 Signals, Sorting, and their related subroutines, as well as
1197 general subroutine traps. Includes some OS-Specific traps.
1203 Barewords that used to look like strings to Perl will now look like subroutine
1204 calls if a subroutine by that name is defined before the compiler sees them.
1206 sub SeeYa { warn"Hasta la vista, baby!" }
1207 $SIG{'TERM'} = SeeYa;
1208 print "SIGTERM is now $SIG{'TERM'}\n";
1210 # perl4 prints: SIGTERM is main'SeeYa
1211 # perl5 prints: SIGTERM is now main::1
1213 Use B<-w> to catch this one
1215 =item * (Sort Subroutine)
1217 reverse is no longer allowed as the name of a sort subroutine.
1219 sub reverse{ print "yup "; $a <=> $b }
1220 print sort reverse a,b,c;
1222 # perl4 prints: yup yup yup yup abc
1225 =item * warn() won't let you specify a filehandle.
1227 Although it _always_ printed to STDERR, warn() would let you specify a
1228 filehandle in perl4. With perl5 it does not.
1232 # perl4 prints: Foo!
1233 # perl5 prints: String found where operator expected
1243 Under HPUX, and some other SysV OSes, one had to reset any signal handler,
1244 within the signal handler function, each time a signal was handled with
1245 perl4. With perl5, the reset is now done correctly. Any code relying
1246 on the handler _not_ being reset will have to be reworked.
1248 Since version 5.002, Perl uses sigaction() under SysV.
1253 $SIG{'INT'} = 'gotit';
1262 while (1) {sleep(10);}
1265 # perl4 (HPUX) prints: Got INT...
1266 # perl5 (HPUX) prints: Got INT... Got INT...
1270 Under SysV OSes, C<seek()> on a file opened to append C<<< >> >>> now does
1271 the right thing w.r.t. the fopen() manpage. e.g., - When a file is opened
1272 for append, it is impossible to overwrite information already in
1275 open(TEST,">>seek.test");
1276 $start = tell TEST ;
1281 seek(TEST,$start,0);
1282 print TEST "18 characters here";
1284 # perl4 (solaris) seek.test has: 18 characters here
1285 # perl5 (solaris) seek.test has: 1 2 3 4 5 6 7 8 9 18 characters here
1291 =head2 Interpolation Traps
1293 Perl4-to-Perl5 traps having to do with how things get interpolated
1294 within certain expressions, statements, contexts, or whatever.
1298 =item * Interpolation
1300 @ now always interpolates an array in double-quotish strings.
1302 print "To: someone@somewhere.com\n";
1304 # perl4 prints: To:someone@somewhere.com
1305 # perl < 5.6.1, error : In string, @somewhere now must be written as \@somewhere
1306 # perl >= 5.6.1, warning : Possible unintended interpolation of @somewhere in string
1308 =item * Interpolation
1310 Double-quoted strings may no longer end with an unescaped $ or @.
1314 print "foo is $foo, bar is $bar\n";
1316 # perl4 prints: foo is foo$, bar is bar@
1317 # perl5 errors: Final $ should be \$ or $name
1319 Note: perl5 DOES NOT error on the terminating @ in $bar
1321 =item * Interpolation
1323 Perl now sometimes evaluates arbitrary expressions inside braces that occur
1324 within double quotes (usually when the opening brace is preceded by C<$>
1330 sub foo { return "bar" };
1331 print "|@{w.w.w}|${main'foo}|";
1333 # perl4 prints: |@{w.w.w}|foo|
1334 # perl5 prints: |buz|bar|
1336 Note that you can C<use strict;> to ward off such trappiness under perl5.
1338 =item * Interpolation
1340 The construct "this is $$x" used to interpolate the pid at that
1341 point, but now apparently tries to dereference $x. C<$$> by itself still
1342 works fine, however.
1344 print "this is $$x\n";
1346 # perl4 prints: this is XXXx (XXX is the current pid)
1347 # perl5 prints: this is
1349 =item * Interpolation
1351 Creation of hashes on the fly with C<eval "EXPR"> now requires either both
1352 C<$>'s to be protected in the specification of the hash name, or both curlies
1353 to be protected. If both curlies are protected, the result will be compatible
1354 with perl4 and perl5. This is a very common practice, and should be changed
1355 to use the block form of C<eval{}> if possible.
1357 $hashname = "foobar";
1360 eval "\$$hashname{'$key'} = q|$value|";
1361 (defined($foobar{'baz'})) ? (print "Yup") : (print "Nope");
1364 # perl5 prints: Nope
1368 eval "\$$hashname{'$key'} = q|$value|";
1372 eval "\$\$hashname{'$key'} = q|$value|";
1374 causes the following result:
1376 # perl4 prints: Nope
1381 eval "\$$hashname\{'$key'\} = q|$value|";
1383 causes the following result:
1387 # and is compatible for both versions
1390 =item * Interpolation
1392 perl4 programs which unconsciously rely on the bugs in earlier perl versions.
1394 perl -e '$bar=q/not/; print "This is $foo{$bar} perl5"'
1396 # perl4 prints: This is not perl5
1397 # perl5 prints: This is perl5
1399 =item * Interpolation
1401 You also have to be careful about array references.
1406 perl 5 prints: syntax error
1408 =item * Interpolation
1410 Similarly, watch out for:
1413 print "\$$foo{bar}\n";
1415 # perl4 prints: $array{bar}
1418 Perl 5 is looking for C<$array{bar}> which doesn't exist, but perl 4 is
1419 happy just to expand $foo to "array" by itself. Watch out for this
1420 especially in C<eval>'s.
1422 =item * Interpolation
1424 C<qq()> string passed to C<eval>
1427 foreach \$y (keys %\$x\) {
1432 # perl4 runs this ok
1433 # perl5 prints: Can't find string terminator ")"
1445 Existing dbm databases created under perl4 (or any other dbm/ndbm tool)
1446 may cause the same script, run under perl5, to fail. The build of perl5
1447 must have been linked with the same dbm/ndbm as the default for C<dbmopen()>
1448 to function properly without C<tie>'ing to an extension dbm implementation.
1450 dbmopen (%dbm, "file", undef);
1454 # perl5 prints: ok (IFF linked with -ldbm or -lndbm)
1459 Existing dbm databases created under perl4 (or any other dbm/ndbm tool)
1460 may cause the same script, run under perl5, to fail. The error generated
1461 when exceeding the limit on the key/value size will cause perl5 to exit
1464 dbmopen(DB, "testdb",0600) || die "couldn't open db! $!";
1465 $DB{'trap'} = "x" x 1024; # value too large for most dbm/ndbm
1469 dbm store returned -1, errno 28, key "trap" at - line 3.
1473 dbm store returned -1, errno 28, key "trap" at - line 3.
1477 =head2 Unclassified Traps
1483 =item * C<require>/C<do> trap using returned value
1485 If the file doit.pl has:
1493 And the do.pl file has the following single line:
1497 Running doit.pl gives the following:
1499 # perl 4 prints: 3 (aborts the subroutine early)
1502 Same behavior if you replace C<do> with C<require>.
1504 =item * C<split> on empty string with LIMIT specified
1507 @list = split(/foo/, $string, 2)
1509 Perl4 returns a one element list containing the empty string but Perl5
1510 returns an empty list.
1514 As always, if any of these are ever officially declared as bugs,
1515 they'll be fixed and removed.