perltrap - Perl traps for the unwary


The biggest trap of all is forgetting to use the -w switch; see the perlrun manpage . The second biggest trap is not making your entire program runnable under use strict.

Awk Traps

Accustomed awk users should take special note of the following:

C Traps

Cerebral C programmers should take note of the following:

Sed Traps

Seasoned sed programmers should take note of the following:

Shell Traps

Sharp shell programmers should take note of the following:

Perl Traps

Practicing Perl Programmers should take note of the following:

Perl4 to Perl5 Traps

Practicing Perl4 Programmers should take note of the following Perl4-to-Perl5 specific traps.

They're crudely ordered according to the following list:

Discontinuance, Deprecation, and BugFix traps
Anything that's been fixed as a perl4 bug, removed as a perl4 feature or deprecated as a perl4 feature with the intent to encourage usage of some other perl5 feature.

Parsing Traps
Traps that appear to stem from the new parser.

Numerical Traps
Traps having to do with numerical or mathematical operators.

General data type traps
Traps involving perl standard data types.

Context Traps - scalar, list contexts
Traps related to context within lists, scalar statements/declarations.

Precedence Traps
Traps related to the precedence of parsing, evaluation, and execution of code.

General Regular Expression Traps using s///, etc.
Traps related to the use of pattern matching.

Subroutine, Signal, Sorting Traps
Traps related to the use of signals and signal handlers, general subroutines, and sorting, along with sorting subroutines.

OS Traps
OS-specific traps.

DBM Traps
Traps specific to the use of dbmopen(), and specific dbm implementations.

Unclassified Traps
Everything else.

If you find an example of a conversion trap that is not listed here,please submit it to Bill Middleton for inclusion. Also note that at least some of these can be caught with -w.

Discontinuance, Deprecation, and BugFix traps

Anything that has been discontinued, deprecated, or fixed as a bug from perl4.

* Discontinuance
Symbols starting with ``_'' are no longer forced into package main, except for $_ itself (and @_, etc.).

package test; $_legacy = 1; package main; print "\$_legacy is ",$_legacy,"\n"; # perl4 prints: $_legacy is 1 # perl5 prints: $_legacy is

* Deprecation
Double-colon is now a valid package separator in a variable name. Thus these behave differently in perl4 vs. perl5, since the packages don't exist.

$a=1;$b=2;$c=3;$var=4; print "$a::$b::$c "; print "$var::abc::xyz\n"; # perl4 prints: 1::2::3 4::abc::xyz # perl5 prints: 3

Given that :: is now the preferred package delimiter, it is debatable whether this should be classed as a bug or not. (The older package delimiter, ' ,is used here)

$x = 10 ; print "x=${'x}\n" ; # perl4 prints: x=10 # perl5 prints: Can't find string terminator "'" anywhere before EOF

Also see precedence traps, for parsing $:.

* BugFix
The second and third arguments of splice() are now evaluated in scalar context (as the Camel says) rather than list context.

sub sub1{return(0,2) } # return a 2-elem array sub sub2{ return(1,2,3)} # return a 3-elem array @a1 = ("a","b","c","d","e"); @a2 = splice(@a1,&sub1,&sub2); print join(' ',@a2),"\n"; # perl4 prints: a b # perl5 prints: c d e

* Discontinuance
You can't do a goto into a block that is optimized away. Darn.

goto marker1; for(1){ marker1: print "Here I is!\n"; } # perl4 prints: Here I is! # perl5 dumps core (SEGV)

* Discontinuance
It is no longer syntactically legal to use whitespace as the name of a variable, or as a delimiter for any kind of quote construct. Double darn.

$a = ("foo bar"); $b = q baz ; print "a is $a, b is $b\n"; # perl4 prints: a is foo bar, b is baz # perl5 errors: Bare word found where operator expected

* Discontinuance
The archaic while/if BLOCK BLOCK syntax is no longer supported.

if { 1 } { print "True!"; } else { print "False!"; } # perl4 prints: True! # perl5 errors: syntax error at line 1, near "if {"

* BugFix
The ** operator now binds more tightly than unary minus. It was documented to work this way before, but didn't.

print -4**2,"\n"; # perl4 prints: 16 # perl5 prints: -16

* Discontinuance
The meaning of foreach{} has changed slightly when it is iterating over a list which is not an array. This used to assign the list to a temporary array, but no longer does so (for efficiency). This means that you'll now be iterating over the actual values, not over copies of the values. Modifications to the loop variable can change the original values.

@list = ('ab','abc','bcd','def'); foreach $var (grep(/ab/,@list)){ $var = 1; } print (join(':',@list)); # perl4 prints: ab:abc:bcd:def # perl5 prints: 1:1:bcd:def

To retain Perl4 semantics you need to assign your list explicitly to a temporary array and then iterate over that. For example, you might need to change

foreach $var (grep(/ab/,@list)){


foreach $var (@tmp = grep(/ab/,@list)){

Otherwise changing $var will clobber the values of @list. (This most often happens when you use $_ for the loop variable, and call subroutines in the loop that don't properly localize $_.)

* Discontinuance
split with no arguments now behaves like split ' ' (which doesn't return an initial null field if $_ starts with whitespace), it used to behave like split /\s+/ (which does).

$_ = ' hi mom'; print join(':', split); # perl4 prints: :hi:mom # perl5 prints: hi:mom

* Deprecation
Some error messages will be different.

* Discontinuance
Some bugs may have been inadvertently removed. :-)

Parsing Traps

Perl4-to-Perl5 traps from having to do with parsing.

* Parsing
Note the space between . and =

$string . = "more string"; print $string; # perl4 prints: more string # perl5 prints: syntax error at - line 1, near ". ="

* Parsing
Better parsing in perl 5

sub foo {} &foo print("hello, world\n"); # perl4 prints: hello, world # perl5 prints: syntax error

* Parsing
``if it looks like a function, it is a function'' rule.

print ($foo == 1) ? "is one\n" : "is zero\n"; # perl4 prints: is zero # perl5 warns: "Useless use of a constant in void context" if using -w

Numerical Traps

Perl4-to-Perl5 traps having to do with numerical operators, operands, or output from same.

* Numerical
Formatted output and significant digits

print 7.373504 - 0, "\n"; printf "%20.18f\n", 7.373504 - 0; # Perl4 prints: 7.375039999999996141 7.37503999999999614 # Perl5 prints: 7.373504 7.37503999999999614

* Numerical
This specific item has been deleted. It demonstrated how the autoincrement operator would not catch when a number went over the signed int limit. Fixed in 5.003_04. But always be wary when using large ints. If in doubt:

use Math::BigInt;

* Numerical
Assignment of return values from numeric equality tests does not work in perl5 when the test evaluates to false (0). Logical tests now return an null, instead of 0 $p = ($test == 1); print $p,``\n''; # perl4 prints: 0 # perl5 prints:

Also see the General Regular Expression Traps tests for another example of this new feature...

General data type traps

Perl4-to-Perl5 traps involving most data-types, and their usage within certain expressions and/or context.

* (Arrays)
Negative array subscripts now count from the end of the array.

@a = (1, 2, 3, 4, 5); print "The third element of the array is $a[3] also expressed as $a[-2] \n"; # perl4 prints: The third element of the array is 4 also expressed as # perl5 prints: The third element of the array is 4 also expressed as 4

* (Arrays)
Setting $#array lower now discards array elements, and makes them impossible to recover.

@a = (a,b,c,d,e); print "Before: ",join('',@a); $#a =1; print ", After: ",join('',@a); $#a =3; print ", Recovered: ",join('',@a),"\n"; # perl4 prints: Before: abcde, After: ab, Recovered: abcd # perl5 prints: Before: abcde, After: ab, Recovered: ab

* (Hashes)
Hashes get defined before use

local($s,@a,%h); die "scalar \$s defined" if defined($s); die "array \@a defined" if defined(@a); die "hash \%h defined" if defined(%h); # perl4 prints: # perl5 dies: hash %h defined

* (Globs)
glob assignment from variable to variable will fail if the assigned variable is localized subsequent to the assignment

@a = ("This is Perl 4"); *b = *a; local(@a); print @b,"\n"; # perl4 prints: This is Perl 4 # perl5 prints: # Another example *fred = *barney; # fred is aliased to barney @barney = (1, 2, 4); # @fred; print "@fred"; # should print "1, 2, 4" # perl4 prints: 1 2 4 # perl5 prints: Literal @fred now requires backslash

* (Scalar String)
Changes in unary negation (of strings) This change effects both the return value and what it does to auto(magic)increment.

$x = "aaa"; print ++$x," : "; print -$x," : "; print ++$x,"\n"; # perl4 prints: aab : -0 : 1 # perl5 prints: aab : -aab : aac

* (Constants)
perl 4 lets you modify constants:

$foo = "x"; &mod($foo); for ($x = 0; $x < 3; $x++) { &mod("a"); } sub mod { print "before: $_[0]"; $_[0] = "m"; print " after: $_[0]\n"; } # perl4: # before: x after: m # before: a after: m # before: m after: m # before: m after: m # Perl5: # before: x after: m # Modification of a read-only value attempted at line 12. # before: a

* (Scalars)
The behavior is slightly different for:

print "$x", defined $x # perl 4: 1 # perl 5: <no output, $x is not called into existence>

* (Variable Suicide)
Variable suicide behavior is more consistent under Perl 5. Perl5 exhibits the same behavior for associative arrays and scalars, that perl4 exhibits only for scalars.

$aGlobal{ "aKey" } = "global value"; print "MAIN:", $aGlobal{"aKey"}, "\n"; $GlobalLevel = 0; &test( *aGlobal ); sub test { local( *theArgument ) = @_; local( %aNewLocal ); # perl 4 != 5.001l,m $aNewLocal{"aKey"} = "this should never appear"; print "SUB: ", $theArgument{"aKey"}, "\n"; $aNewLocal{"aKey"} = "level $GlobalLevel"; # what should print $GlobalLevel++; if( $GlobalLevel<4 ) { &test( *aNewLocal ); } } # Perl4: # MAIN:global value # SUB: global value # SUB: level 0 # SUB: level 1 # SUB: level 2 # Perl5: # MAIN:global value # SUB: global value # SUB: this should never appear # SUB: this should never appear # SUB: this should never appear

Context Traps - scalar, list contexts

* (list context)
The elements of argument lists for formats are now evaluated in list context. This means you can interpolate list values now.

    @fmt = (``foo'',``bar'',``baz'');
    format STDOUT=
    @<<<<< @||||| @>>>>>
    # perl4 errors:  Please use commas to separate fields in file
    # perl5 prints: foo     bar      baz

* (scalar context)
The caller() function now returns a false value in a scalar context if there is no caller. This lets library files determine if they're being required.

caller() ? (print "You rang?\n") : (print "Got a 0\n"); # perl4 errors: There is no caller # perl5 prints: Got a 0

* (scalar context)
The comma operator in a scalar context is now guaranteed to give a scalar context to its arguments.

@y= ('a','b','c'); $x = (1, 2, @y); print "x = $x\n"; # Perl4 prints: x = c # Thinks list context interpolates list # Perl5 prints: x = 3 # Knows scalar uses length of list

* (list, builtin)
sprintf() funkiness (array argument converted to scalar array count) This test could be added to t/op/sprintf.t

@z = ('%s%s', 'foo', 'bar'); $x = sprintf(@z); if ($x eq 'foobar') {print "ok 2\n";} else {print "not ok 2 '$x'\n";} # perl4 prints: ok 2 # perl5 prints: not ok 2

printf() works fine, though:

printf STDOUT (@z); print "\n"; # perl4 prints: foobar # perl5 prints: foobar

Probably a bug.

Precedence Traps

Perl4-to-Perl5 traps involving precedence order.

* Precedence
LHS vs. RHS when both sides are getting an op.

@arr = ( 'left', 'right' ); $a{shift @arr} = shift @arr; print join( ' ', keys %a ); # perl4 prints: left # perl5 prints: right

* Precedence
These are now semantic errors because of precedence:

@list = (1,2,3,4,5); %map = ("a",1,"b",2,"c",3,"d",4); $n = shift @list + 2; # first item in list plus 2 print "n is $n, "; $m = keys %map + 2; # number of items in hash plus 2 print "m is $m\n"; # perl4 prints: n is 3, m is 6 # perl5 errors and fails to compile

* Precedence
The precedence of assignment operators is now the same as the precedence of assignment. Perl 4 mistakenly gave them the precedence of the associated operator. So you now must parenthesize them in expressions like

/foo/ ? ($a += 2) : ($a -= 2); Otherwise /foo/ ? $a += 2 : $a -= 2

would be erroneously parsed as

(/foo/ ? $a += 2 : $a) -= 2;

On the other hand,

$a += /foo/ ? 1 : 2;

now works as a C programmer would expect.

* Precedence
open FOO || die;

is now incorrect. You need parens around the filehandle. Otherwise, perl5 leaves the statement as it's default precedence:

open(FOO || die); # perl4 opens or dies # perl5 errors: Precedence problem: open FOO should be open(FOO)

* Precedence
perl4 gives the special variable, $: precedence, where perl5 treats $:: as main package

$a = "x"; print "$::a"; # perl 4 prints: -:a # perl 5 prints: x

* Precedence
concatenation precedence over filetest operator?

-e $foo .= "q" # perl4 prints: no output # perl5 prints: Can't modify -e in concatenation

* Precedence
Assignment to value takes precedence over assignment to key in perl5 when using the shift operator on both sides.

@arr = ( 'left', 'right' ); $a{shift @arr} = shift @arr; print join( ' ', keys %a ); # perl4 prints: left # perl5 prints: right

General Regular Expression Traps using s///, etc.

All types of RE traps.

* Regular Expression
s'$lhs'$rhs' now does no interpolation on either side. It used to interpolate $lhs but not $rhs. (And still does not match a literal '$' in string)

$a=1;$b=2; $string = '1 2 $a $b'; $string =~ s'$a'$b'; print $string,"\n"; # perl4 prints: $b 2 $a $b # perl5 prints: 1 2 $a $b

* Regular Expression
m//g now attaches its state to the searched string rather than the regular expression. (Once the scope of a block is left for the sub, the state of the searched string is lost)

$_ = "ababab"; while(m/ab/g){ &doit("blah"); } sub doit{local($_) = shift; print "Got $_ "} # perl4 prints: blah blah blah # perl5 prints: infinite loop blah...

* Regular Expression
If no parentheses are used in a match, Perl4 sets $+ to the whole match, just like $&. Perl5 does not.

"abcdef" =~ /b.*e/; print "\$+ = $+\n"; # perl4 prints: bcde # perl5 prints:

* Regular Expression
substitution now returns the null string if it fails

$string = "test"; $value = ($string =~ s/foo//); print $value, "\n"; # perl4 prints: 0 # perl5 prints:

Also see Numerical Traps for another example of this new feature.

* Regular Expression
s`lhs`rhs` (using backticks) is now a normal substitution, with no backtick expansion

$string = ""; $string =~ s`^`hostname`; print $string, "\n"; # perl4 prints: <the local hostname> # perl5 prints: hostname

* Regular Expression
Stricter parsing of variables used in regular expressions

s/^([^$grpc]*$grpc[$opt$plus$rep]?)//o; # perl4: compiles w/o error # perl5: with Scalar found where operator expected ..., near "$opt$plus"

an added component of this example, apparently from the same script, is the actual value of the s'd string after the substitution. [$opt] is a character class in perl4 and an array subscript in perl5

$grpc = 'a'; $opt = 'r'; $_ = 'bar'; s/^([^$grpc]*$grpc[$opt]?)/foo/; print ; # perl4 prints: foo # perl5 prints: foobar

* Regular Expression
Under perl5, m?x? matches only once, like ?x?. Under perl4, it matched repeatedly, like /x/ or m!x!.

$test = "once"; sub match { $test =~ m?once?; } &match(); if( &match() ) { # m?x? matches more then once print "perl4\n"; } else { # m?x? matches only once print "perl5\n"; } # perl4 prints: perl4 # perl5 prints: perl5

Subroutine, Signal, Sorting Traps

The general group of Perl4-to-Perl5 traps having to do with Signals, Sorting, and their related subroutines, as well as general subroutine traps. Includes some OS-Specific traps.

* (Signals)
Barewords that used to look like strings to Perl will now look like subroutine calls if a subroutine by that name is defined before the compiler sees them.

sub SeeYa { warn"Hasta la vista, baby!" } $SIG{'TERM'} = SeeYa; print "SIGTERM is now $SIG{'TERM'}\n"; # perl4 prints: SIGTERM is main'SeeYa # perl5 prints: SIGTERM is now main::1

Use -w to catch this one

* (Sort Subroutine)
reverse is no longer allowed as the name of a sort subroutine.

sub reverse{ print "yup "; $a <=> $b } print sort reverse a,b,c; # perl4 prints: yup yup yup yup abc # perl5 prints: abc

* warn() won't let you specify a filehandle.
Although it _always_ printed to STDERR, warn() would let you specify a filehandle in perl4. With perl5 it does not.

warn STDERR "Foo!"; # perl4 prints: Foo! # perl5 prints: String found where operator expected

OS Traps

* (SysV)
Under HPUX, and some other SysV OS's, one had to reset any signal handler, within the signal handler function, each time a signal was handled with perl4. With perl5, the reset is now done correctly. Any code relying on the handler _not_ being reset will have to be reworked.

5.002 and beyond uses sigaction() under SysV

sub gotit { print "Got @_... "; } $SIG{'INT'} = 'gotit'; $| = 1; $pid = fork; if ($pid) { kill('INT', $pid); sleep(1); kill('INT', $pid); } else { while (1) {sleep(10);} } # perl4 (HPUX) prints: Got INT... # perl5 (HPUX) prints: Got INT... Got INT...

* (SysV)
Under SysV OS's, seek() on a file opened to append >> now does the right thing w.r.t. the fopen() man page. e.g. - When a file is opened for append, it is impossible to overwrite information already in the file.

    $start = tell TEST ;  
    foreach(1 .. 9){
        print TEST ``$_ '';
    $end = tell TEST ;
    print TEST ``18 characters here'';
    # perl4 (solaris) seek.test has: 18 characters here
    # perl5 (solaris) seek.test has: 1 2 3 4 5 6 7 8 9 18 characters here

Interpolation Traps

Perl4-to-Perl5 traps having to do with how things get interpolated within certain expressions, statements, contexts, or whatever.

* Interpolation
@ now always interpolates an array in double-quotish strings.

print "To:\n"; # perl4 prints: # perl5 errors : Literal @somewhere now requires backslash

* Interpolation
Double-quoted strings may no longer end with an unescaped $ or @.

$foo = "foo$"; $bar = "bar@"; print "foo is $foo, bar is $bar\n"; # perl4 prints: foo is foo$, bar is bar@ # perl5 errors: Final $ should be \$ or $name

Note: perl5 DOES NOT error on the terminating @ in $bar

* Interpolation
Perl now sometimes evaluates arbitrary expressions inside braces that occur within double quotes (usually when the opening brace is preceded by $ or @).

@www = "buz"; $foo = "foo"; $bar = "bar"; sub foo { return "bar" }; print "|@{w.w.w}|${main'foo}|"; # perl4 prints: |@{w.w.w}|foo| # perl5 prints: |buz|bar|

Note that you can use strict; to ward off such trappiness under perl5.

* Interpolation
The construct ``this is $$x'' used to interpolate the pid at that point, but now apparently tries to dereference $x. $$ by itself still works fine, however.

print "this is $$x\n"; # perl4 prints: this is XXXx (XXX is the current pid) # perl5 prints: this is

* Interpolation
Creation of hashes on the fly with eval ``EXPR'' now requires either both $'s to be protected in the specification of the hash name, or both curlies to be protected. If both curlies are protected, the result will be compatible with perl4 and perl5. This is a very common practice, and should be changed to use the block form of eval{} if possible.

$hashname = "foobar"; $key = "baz"; $value = 1234; eval "\$$hashname{'$key'} = q|$value|"; (defined($foobar{'baz'})) ? (print "Yup") : (print "Nope"); # perl4 prints: Yup # perl5 prints: Nope


eval "\$$hashname{'$key'} = q|$value|";


eval "\$\$hashname{'$key'} = q|$value|";

causes the following result:

# perl4 prints: Nope # perl5 prints: Yup

or, changing to

eval "\$$hashname\{'$key'\} = q|$value|";

causes the following result:

# perl4 prints: Yup # perl5 prints: Yup # and is compatible for both versions

* Interpolation
perl4 programs which unconsciously rely on the bugs in earlier perl versions.

perl -e '$bar=q/not/; print "This is $foo{$bar} perl5"' # perl4 prints: This is not perl5 # perl5 prints: This is perl5

* Interpolation
You also have to be careful about array references.

print "$foo{" perl 4 prints: { perl 5 prints: syntax error

* Interpolation
Similarly, watch out for:

$foo = "array"; print "\$$foo{bar}\n"; # perl4 prints: $array{bar} # perl5 prints: $

Perl 5 is looking for $array{bar} which doesn't exist, but perl 4 is happy just to expand $foo to ``array'' by itself. Watch out for this especially in eval's.

* Interpolation
qq() string passed to eval

eval qq( foreach \$y (keys %\$x\) { \$count++; } ); # perl4 runs this ok # perl5 prints: Can't find string terminator ")"

DBM Traps

General DBM traps.

Existing dbm databases created under perl4 (or any other dbm/ndbm tool) may cause the same script, run under perl5, to fail. The build of perl5 must have been linked with the same dbm/ndbm as the default for dbmopen() to function properly without tie'ing to an extension dbm implementation.

dbmopen (%dbm, "file", undef); print "ok\n"; # perl4 prints: ok # perl5 prints: ok (IFF linked with -ldbm or -lndbm)

Existing dbm databases created under perl4 (or any other dbm/ndbm tool) may cause the same script, run under perl5, to fail. The error generated when exceeding the limit on the key/value size will cause perl5 to exit immediately.

dbmopen(DB, "testdb",0600) || die "couldn't open db! $!"; $DB{'trap'} = "x" x 1024; # value too large for most dbm/ndbm print "YUP\n"; # perl4 prints: dbm store returned -1, errno 28, key "trap" at - line 3. YUP # perl5 prints: dbm store returned -1, errno 28, key "trap" at - line 3.

Unclassified Traps

Everything else.

* Unclassified
require/do trap using returned value

If the file has:

sub foo { $rc = do "./"; return 8; } print &foo, "\n";

And the file has the following single line:

return 3;

Running gives the following:

# perl 4 prints: 3 (aborts the subroutine early) # perl 5 prints: 8

Same behavior if you replace do with require.

As always, if any of these are ever officially declared as bugs, they'll be fixed and removed.