perltrap - Perl traps for the unwary
The biggest trap of all is forgetting to use warnings
or use the -w
switch; see perllexwarn and perlrun. The second biggest trap is not
making your entire program runnable under use strict
. The third biggest
trap is not reading the list of changes in this version of Perl; see
perldelta.
Accustomed awk users should take special note of the following:
A Perl program executes only once, not once for each input line. You can
do an implicit loop with -n
or -p
.
The English module, loaded via
- use English;
allows you to refer to special variables (like $/
) with names (like
$RS), as though they were in awk; see perlvar for details.
Semicolons are required after all simple statements in Perl (except at the end of a block). Newline is not a statement delimiter.
Curly brackets are required on if
s and while
s.
Variables begin with "$", "@" or "%" in Perl.
Arrays index from 0. Likewise string positions in substr() and index().
You have to decide whether your array has numeric or string indices.
Hash values do not spring into existence upon mere reference.
You have to decide whether you want to use string or numeric comparisons.
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 awk's.
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 perlvar.
$<digit> does not refer to fields--it refers to substrings matched by the last match pattern.
The print() statement does not add field and record separators unless
you set $,
and $\
. You can set $OFS and $ORS if you're using
the English module.
You must open your files before you print to them.
The range operator is "..", not comma. The comma operator works as in C.
The match operator is "=~", not "~". ("~" is the one's complement operator, as in C.)
The exponentiation operator is "**", not "^". "^" is the XOR operator, as in C. (You know, one could get the feeling that awk is basically incompatible with C.)
The concatenation operator is ".", not the null string. (Using the
null string would render /pat/ /pat/
unparsable, because the third slash
would be interpreted as a division operator--the tokenizer is in fact
slightly context sensitive for operators like "/", "?", and ">".
And in fact, "." itself can be the beginning of a number.)
The following variables work differently:
- Awk Perl
- ARGC scalar @ARGV (compare with $#ARGV)
- ARGV[0] $0
- FILENAME $ARGV
- FNR $. - something
- FS (whatever you like)
- NF $#Fld, or some such
- NR $.
- OFMT $#
- OFS $,
- ORS $\
- RLENGTH length($&)
- RS $/
- RSTART length($`)
- SUBSEP $;
You cannot set $RS to a pattern, only a string.
When in doubt, run the awk construct through a2p and see what it gives you.
Cerebral C and C++ programmers should take note of the following:
Curly brackets are required on if
's and while
's.
You must use elsif
rather than else if
.
The break
and continue keywords from C become in Perl last
and next, respectively. Unlike in C, these do not work within a
do { } while
construct. See Loop Control in perlsyn.
The switch statement is called given/when and only available in
perl 5.10 or newer. See Switch statements in perlsyn.
Variables begin with "$", "@" or "%" in Perl.
Comments begin with "#", not "/*" or "//". Perl may interpret C/C++ comments as division operators, unterminated regular expressions or the defined-or operator.
You can't take the address of anything, although a similar operator in Perl is the backslash, which creates a reference.
ARGV
must be capitalized. $ARGV[0]
is C's argv[1]
, and argv[0]
ends up in $0
.
System calls such as link(), unlink(), rename(), etc. return nonzero for success, not 0. (system(), however, returns zero for success.)
Signal handlers deal with signal names, not numbers. Use kill -l
to find their names on your system.
Seasoned sed programmers should take note of the following:
A Perl program executes only once, not once for each input line. You can
do an implicit loop with -n
or -p
.
Backreferences in substitutions use "$" rather than "\".
The pattern matching metacharacters "(", ")", and "|" do not have backslashes in front.
The range operator is ...
, rather than comma.
Sharp shell programmers should take note of the following:
The backtick operator does variable interpolation without regard to the presence of single quotes in the command.
The backtick operator does no translation of the return value, unlike csh.
Shells (especially csh) 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.
Shells interpret scripts a little bit at a time. Perl compiles the
entire program before executing it (except for BEGIN
blocks, which
execute at compile time).
The arguments are available via @ARGV, not $1, $2, etc.
The environment is not automatically made available as separate scalar variables.
The shell's test
uses "=", "!=", "<" etc for string comparisons and "-eq",
"-ne", "-lt" etc for numeric comparisons. This is the reverse of Perl, which
uses eq
, ne
, lt
for string comparisons, and ==
, !=
<
etc
for numeric comparisons.
Practicing Perl Programmers should take note of the following:
Remember that many operations behave differently in a list context than they do in a scalar one. See perldata for details.
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.
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 only be list operators, never unary ones.) See perlop and perlsub.
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.
The <FH> construct is not the name of the filehandle, it is a readline operation on that handle. The data read is assigned to $_ only if the file read is the sole condition in a while loop:
- while (<FH>) { }
- while (defined($_ = <FH>)) { }..
- <FH>; # data discarded!
Remember not to use =
when you need =~
;
these two constructs are quite different:
- $x = /foo/;
- $x =~ /foo/;
The do {}
construct isn't a real loop that you can use
loop control on.
Use my() for local variables whenever you can get away with
it (but see perlform for where you can't).
Using local() actually gives a local value to a global
variable, which leaves you open to unforeseen side-effects
of dynamic scoping.
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.
Practicing Perl4 Programmers should take note of the following Perl4-to-Perl5 specific traps.
They're crudely ordered according to the following list:
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.
Traps that appear to stem from the new parser.
Traps having to do with numerical or mathematical operators.
Traps involving perl standard data types.
Traps related to context within lists, scalar statements/declarations.
Traps related to the precedence of parsing, evaluation, and execution of code.
Traps related to the use of pattern matching.
Traps related to the use of signals and signal handlers, general subroutines, and sorting, along with sorting subroutines.
OS-specific traps.
Traps specific to the use of dbmopen(), and specific dbm implementations.
Everything else.
If you find an example of a conversion trap that is not listed here,
please submit it to <perlbug@perl.org> for inclusion.
Also note that at least some of these can be caught with the
use warnings
pragma or the -w switch.
Anything that has been discontinued, deprecated, or fixed as a bug from perl4.
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
Double-colon is now a valid package separator in a variable name. Thus these behave differently in perl4 vs. perl5, because the packages don't exist.
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
You can avoid this problem, and remain compatible with perl4, if you always explicitly include the package name:
- $x = 10;
- print "x=${main'x}\n";
Also see precedence traps, for parsing $:
.
splice() are now in scalar context
The second and third arguments of splice() are now evaluated in scalar
context (as the Camel says) rather than list context.
goto into a block that is optimized away
You can't do a goto into a block that is optimized away. Darn.
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: Bareword found where operator expected
while/if BLOCK BLOCK gone
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 test.pl line 1, near "if {"
**
binds tighter than unary minus
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
foreach
changed when iterating over a list
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.
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)){
to
- 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 $_
.)
split with no args behavior changed
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).
Perl 4 would ignore any text which was attached to an -e switch, always taking the code snippet from the following arg. Additionally, it would silently accept an -e switch without a following arg. Both of these behaviors have been fixed.
- perl -e'print "attached to -e"' 'print "separate arg"'
- # perl4 prints: separate arg
- # perl5 prints: attached to -e
- perl -e
- # perl4 prints:
- # perl5 dies: No code specified for -e.
push returns number of elements in resulting list
In Perl 4 the return value of push was undocumented, but it was
actually the last value being pushed onto the target list. In Perl 5
the return value of push is documented, but has changed, it is the
number of elements in the resulting list.
Some error messages will be different.
split() honors subroutine args
In Perl 4, if in list context the delimiters to the first argument of
split() were ??
, the result would be placed in @_
as well as
being returned. Perl 5 has more respect for your subroutine arguments.
Some bugs may have been inadvertently removed. :-)
Perl4-to-Perl5 traps from having to do with parsing.
Note the space between . and =
- $string . = "more string";
- print $string;
- # perl4 prints: more string
- # perl5 prints: syntax error at - line 1, near ". ="
Better parsing in perl 5
- sub foo {}
- &foo
- print("hello, world\n");
- # perl4 prints: hello, world
- # perl5 prints: syntax error
"if it looks like a function, it is a function" rule.
- ($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
$#array
differs
String interpolation of the $#array
construct differs when braces
are to used around the name.
- @a = (1..3);
- print "${#a}";
- # perl4 prints: 2
- # perl5 fails with syntax error
- @ = (1..3);
- print "$#{a}";
- # perl4 prints: {a}
- # perl5 prints: 2
map, grep followed by { if it starts BLOCK or hash ref
When perl sees map { (or grep {), it has to guess whether the {
starts a BLOCK or a hash reference. If it guesses wrong, it will report
a syntax error near the } and the missing (or unexpected) comma.
Use unary +
before { on a hash reference, and unary +
applied
to the first thing in a BLOCK (after {), for perl to guess right all
the time. (See map.)
Perl4-to-Perl5 traps having to do with numerical operators, operands, or output from same.
Formatted output and significant digits. In general, Perl 5 tries to be more precise. For example, on a Solaris Sparc:
- print 7.373504 - 0, "\n";
- printf "%20.18f\n", 7.373504 - 0;
- # Perl4 prints:
- 7.3750399999999996141
- 7.375039999999999614
- # Perl5 prints:
- 7.373504
- 7.375039999999999614
Notice how the first result looks better in Perl 5.
Your results may vary, since your floating point formatting routines and even floating point format may be slightly different.
This specific item has been deleted. It demonstrated how the auto-increment operator would not catch when a number went over the signed int limit. Fixed in version 5.003_04. But always be wary when using large integers. If in doubt:
- use Math::BigInt;
Assignment of return values from numeric equality tests does not work in perl5 when the test evaluates to false (0). Logical tests now return a null, instead of 0
- $p = ($test == 1);
- print $p,"\n";
- # perl4 prints: 0
- # perl5 prints:
Also see General Regular Expression Traps using s///, etc. for another example of this new feature...
When bitwise operators which can operate upon either numbers or
strings (& | ^ ~
) are given only strings as arguments, perl4 would
treat the operands as bitstrings so long as the program contained a call
to the vec() function. perl5 treats the string operands as bitstrings.
(See Bitwise String Operators in perlop for more details.)
- $fred = "10";
- $barney = "12";
- $betty = $fred & $barney;
- print "$betty\n";
- # Uncomment the next line to change perl4's behavior
- # ($dummy) = vec("dummy", 0, 0);
- # Perl4 prints:
- 8
- # Perl5 prints:
- 10
- # If vec() is used anywhere in the program, both print:
- 10
Perl4-to-Perl5 traps involving most data-types, and their usage within certain expressions and/or context.
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
$#array
lower now discards array elements
Setting $#array
lower now discards array elements, and makes them
impossible to recover.
Hashes get defined before use
Perl will now generate a warning when it sees defined(@a) and defined(%h).
glob assignment from variable to variable will fail if the assigned variable is localized subsequent to the assignment
undef to glob
Assigning undef to a glob has no effect in Perl 5. In Perl 4
it undefines the associated scalar (but may have other side effects
including SEGVs). Perl 5 will also warn if undef is assigned to a
typeglob. (Note that assigning undef to a typeglob is different
than calling the undef function on a typeglob (undef *foo
), which
has quite a few effects.
Changes in unary negation (of strings) This change effects both the return value and what it does to auto(magic)increment.
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 foo.pl line 12.
- # before: a
defined $var
behavior changed
The behavior is slightly different for:
Variable suicide behavior is more consistent under Perl 5. Perl5 exhibits the same behavior for hashes and scalars, that perl4 exhibits for only 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
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=
- @<<<<< @||||| @>>>>>
- @fmt;
- .
- write;
- # perl4 errors: Please use commas to separate fields in file
- # perl5 prints: foo bar baz
caller() returns false value in scalar context if no caller present
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.
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 = $x\n";
- # Perl4 prints: x = c # Thinks list context interpolates list
- # Perl5 prints: x = 3 # Knows scalar uses length of list
sprintf() prototyped as ($;@)
sprintf() is prototyped as ($;@), so its first argument is given scalar
context. Thus, if passed an array, it will probably not do what you want,
unlike Perl 4:
printf() works the same as it did in Perl 4, though:
- @z = ('%s%s', 'foo', 'bar');
- printf STDOUT (@z);
- # perl4 prints: foobar
- # perl5 prints: foobar
Perl4-to-Perl5 traps involving precedence order.
Perl 4 has almost the same precedence rules as Perl 5 for the operators that they both have. Perl 4 however, seems to have had some inconsistencies that made the behavior differ from what was documented.
LHS vs. RHS of any assignment operator. LHS is evaluated first in perl4, second in perl5; this can affect the relationship between side-effects in sub-expressions.
These are now semantic errors because of precedence:
The precedence of assignment operators is now the same as the precedence of assignment. Perl 4 mistakenly gave them the precedence of the associated operator. So you now must parenthesize them in expressions like
- /foo/ ? ($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.
open requires parentheses around filehandle
is now incorrect. You need parentheses around the filehandle. Otherwise, perl5 leaves the statement as its default precedence:
$:
precedence over $::
gone
perl4 gives the special variable, $:
precedence, where perl5
treats $::
as main package
- $a = "x"; print "$::a";
- # perl 4 prints: -:a
- # perl 5 prints: x
perl4 had buggy precedence for the file test operators vis-a-vis
the assignment operators. Thus, although the precedence table
for perl4 leads one to believe -e $foo .= "q"
should parse as
((-e $foo) .= "q")
, it actually parses as (-e ($foo .= "q"))
.
In perl5, the precedence is as documented.
- -e $foo .= "q"
- # perl4 prints: no output
- # perl5 prints: Can't modify -e in concatenation
keys, each, values are regular named unary operators
In perl4, keys(), each() and values() were special high-precedence operators
that operated on a single hash, but in perl5, they are regular named unary
operators. As documented, named unary operators have lower precedence
than the arithmetic and concatenation operators + - ., but the perl4
variants of these operators actually bind tighter than + - ..
Thus, for:
The perl4 behavior was probably more useful, if less consistent.
All types of RE traps.
s'$lhs'$rhs' interpolates on either side
s'$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 $b
m//g attaches its state to the searched string
m//g now attaches its state to the searched string rather than the
regular expression. (Once the scope of a block is left for the sub, the
state of the searched string is lost)
m//o used within an anonymous sub
Currently, if you use the m//o qualifier on a regular expression
within an anonymous sub, all closures generated from that anonymous
sub will use the regular expression as it was compiled when it was used
the very first time in any such closure. For instance, if you say
- sub build_match {
- my($left,$right) = @_;
- return sub { $_[0] =~ /$left stuff $right/o; };
- }
- $good = build_match('foo','bar');
- $bad = build_match('baz','blarch');
- print $good->('foo stuff bar') ? "ok\n" : "not ok\n";
- print $bad->('baz stuff blarch') ? "ok\n" : "not ok\n";
- print $bad->('foo stuff bar') ? "not ok\n" : "ok\n";
For most builds of Perl5, this will print: ok not ok not ok
build_match() will always return a sub which matches the contents of $left and $right as they were the first time that build_match() was called, not as they are in the current call.
$+
isn't set to whole match
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:
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.
s`lhs`rhs` is now a normal substitution
s`lhs`rhs` (using backticks) is now a normal substitution, with no
backtick expansion
- $string = "";
- $string =~ s`^`hostname`;
- print $string, "\n";
- # perl4 prints: <the local hostname>
- # perl5 prints: hostname
Stricter parsing of variables used in regular expressions
- s/^([^$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
m?x? matches only once
Under perl5, m?x? matches only once, like ?x?
. Under perl4, it matched
repeatedly, like /x/
or m!x!.
Unlike in Ruby, failed matches in Perl do not reset the match variables
($1, $2, ..., $`
, ...).
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.
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.
Use -w to catch this one
reverse is no longer allowed as the name of a sort subroutine.
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
Under HPUX, and some other SysV OSes, 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.
Since version 5.002, Perl uses sigaction() under SysV.
seek() appends correctly
Under SysV OSes, seek() on a file opened to append >>
now does
the right thing w.r.t. the fopen() manpage. e.g., - When a file is opened
for append, it is impossible to overwrite information already in
the file.
Perl4-to-Perl5 traps having to do with how things get interpolated within certain expressions, statements, contexts, or whatever.
@
always interpolates an array in double-quotish strings
@ now always interpolates an array in double-quotish strings.
- print "To: someone@somewhere.com\n";
- # perl4 prints: To:someone@somewhere.com
- # perl < 5.6.1, error : In string, @somewhere now must be written as \@somewhere
- # perl >= 5.6.1, warning : Possible unintended interpolation of @somewhere in string
Double-quoted strings may no longer end with an unescaped $.
- $foo = "foo$";
- print "foo is $foo\n";
- # perl4 prints: foo is foo$
- # perl5 errors: Final $ should be \$ or $name
Note: perl5 DOES NOT error on the terminating @ in $bar
Perl now sometimes evaluates arbitrary expressions inside braces that occur
within double quotes (usually when the opening brace is preceded by $
or @
).
Note that you can use strict;
to ward off such trappiness under perl5.
$$x
now tries to dereference $x
The construct "this is $$x" used to interpolate the pid at that point, but
now tries to dereference $x. $$
by itself still works fine, however.
- $s = "a reference";
- $x = *s;
- print "this is $$x\n";
- # perl4 prints: this is XXXx (XXX is the current pid)
- # perl5 prints: this is a reference
eval "EXPR"
requires protection
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.
Changing
- eval "\$$hashname{'$key'} = q|$value|";
to
- 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
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
You also have to be careful about array and hash brackets during interpolation.
- print "$foo["
- perl 4 prints: [
- perl 5 prints: syntax error
- print "$foo{"
- perl 4 prints: {
- perl 5 prints: syntax error
Perl 5 is expecting to find an index or key name following the respective brackets, as well as an ending bracket of the appropriate type. In order to mimic the behavior of Perl 4, you must escape the bracket like so.
\$$foo{bar}
Similarly, watch out for: \$$foo{bar}
- $foo = "baz";
- print "\$$foo{bar}\n";
- # perl4 prints: $baz{bar}
- # perl5 prints: $
Perl 5 is looking for $foo{bar}
which doesn't exist, but perl 4 is
happy just to expand $foo to "baz" by itself. Watch out for this
especially in eval's.
qq() string passed to eval will not find string terminator
- eval qq(
- foreach \$y (keys %\$x\) {
- \$count++;
- }
- );
- # perl4 runs this ok
- # perl5 prints: Can't find string terminator ")"
General DBM traps.
dbmopen()
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.
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.
Everything else.
require/do trap using returned value
If the file doit.pl has:
And the do.pl file has the following single line:
- return 3;
Running doit.pl gives the following:
- # perl 4 prints: 3 (aborts the subroutine early)
- # perl 5 prints: 8
split on empty string with LIMIT specified
- $string = '';
- @list = split(/foo/, $string, 2)
Perl4 returns a one element list containing the empty string but Perl5 returns an empty list.
As always, if any of these are ever officially declared as bugs, they'll be fixed and removed.