=head1 DESCRIPTION
-The biggest trap of all is forgetting to use the B<-w> switch; see
-L<perlrun>. The second biggest trap is not making your entire program
-runnable under C<use strict>. The third biggest trap is not reading
-the list of changes in this version of Perl; see L<perldelta>.
+The biggest trap of all is forgetting to C<use warnings> or use the B<-w>
+switch; see L<perllexwarn> and L<perlrun>. The second biggest trap is not
+making your entire program runnable under C<use strict>. The third biggest
+trap is not reading the list of changes in this version of Perl; see
+L<perldelta>.
=head2 Awk Traps
=item *
+A Perl program executes only once, not once for each input line. You can
+do an implicit loop with C<-n> or C<-p>.
+
+=item *
+
The English module, loaded via
use English;
The following variables work differently:
Awk Perl
- ARGC $#ARGV or scalar @ARGV
+ ARGC scalar @ARGV (compare with $#ARGV)
ARGV[0] $0
FILENAME $ARGV
FNR $. - something
=back
-=head2 C Traps
+=head2 C/C++ Traps
-Cerebral C programmers should take note of the following:
+Cerebral C and C++ programmers should take note of the following:
=over 4
=item *
-The C<break> and C<continue> keywords from C become in
-Perl C<last> and C<next>, respectively.
-Unlike in C, these do I<not> work within a C<do { } while> construct.
+The C<break> and C<continue> keywords from C become in Perl C<last>
+and C<next>, respectively. Unlike in C, these do I<not> work within a
+C<do { } while> construct. See L<perlsyn/"Loop Control">.
=item *
-There's no switch statement. (But it's easy to build one on the fly.)
+There's no switch statement. (But it's easy to build one on the fly,
+see L<perlsyn/"Basic BLOCKs and Switch Statements">)
=item *
=item *
-Comments begin with "#", not "/*".
+Comments begin with "#", not "/*" or "//". Perl may interpret C/C++
+comments as division operators, unterminated regular expressions or
+the defined-or operator.
=item *
=item *
System calls such as link(), unlink(), rename(), etc. return nonzero for
-success, not 0.
+success, not 0. (system(), however, returns zero for success.)
=item *
=item *
+A Perl program executes only once, not once for each input line. You can
+do an implicit loop with C<-n> or C<-p>.
+
+=item *
+
Backreferences in substitutions use "$" rather than "\".
=item *
The environment is not automatically made available as separate scalar
variables.
+=item *
+
+The shell's C<test> uses "=", "!=", "<" etc for string comparisons and "-eq",
+"-ne", "-lt" etc for numeric comparisons. This is the reverse of Perl, which
+uses C<eq>, C<ne>, C<lt> for string comparisons, and C<==>, C<!=> C<< < >> etc
+for numeric comparisons.
+
=back
=head2 Perl Traps
You cannot discern from mere inspection which builtins
are unary operators (like chop() and chdir())
and which are list operators (like print() and unlink()).
-(User-defined subroutines can be B<only> list operators, never
-unary ones.) See L<perlop>.
+(Unless prototyped, user-defined subroutines can B<only> be list
+operators, never unary ones.) See L<perlop> and L<perlsub>.
=item *
=over 4
-=item * Discontinuance
+=item * Symbols starting with "_" no longer forced into main
Symbols starting with "_" are no longer forced into package main, except
for C<$_> itself (and C<@_>, etc.).
# perl4 prints: $_legacy is 1
# perl5 prints: $_legacy is
-=item * Deprecation
+=item * Double-colon valid package separator in variable name
Double-colon is now a valid package separator in a variable name. Thus these
behave differently in perl4 vs. perl5, because the packages don't exist.
Also see precedence traps, for parsing C<$:>.
-=item * BugFix
+=item * 2nd and 3rd args to C<splice()> are now in scalar context
The second and third arguments of C<splice()> are now evaluated in scalar
context (as the Camel says) rather than list context.
# perl4 prints: a b
# perl5 prints: c d e
-=item * Discontinuance
+=item * Can't do C<goto> into a block that is optimized away
You can't do a C<goto> into a block that is optimized away. Darn.
}
# perl4 prints: Here I is!
- # perl5 dumps core (SEGV)
+ # perl5 errors: Can't "goto" into the middle of a foreach loop
-=item * Discontinuance
+=item * Can't use whitespace as variable name or quote delimiter
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.
# perl4 prints: a is foo bar, b is baz
# perl5 errors: Bareword found where operator expected
-=item * Discontinuance
+=item * C<while/if BLOCK BLOCK> gone
The archaic while/if BLOCK BLOCK syntax is no longer supported.
# perl4 prints: True!
# perl5 errors: syntax error at test.pl line 1, near "if {"
-=item * BugFix
+=item * C<**> binds tighter than unary minus
The C<**> operator now binds more tightly than unary minus.
It was documented to work this way before, but didn't.
# perl4 prints: 16
# perl5 prints: -16
-=item * Discontinuance
+=item * C<foreach> changed when iterating over a list
The meaning of C<foreach{}> has changed slightly when it is iterating over a
list which is not an array. This used to assign the list to a
happens when you use C<$_> for the loop variable, and call subroutines in
the loop that don't properly localize C<$_>.)
-=item * Discontinuance
+=item * C<split> with no args behavior changed
C<split> with no arguments now behaves like C<split ' '> (which doesn't
return an initial null field if $_ starts with whitespace), it used to
# perl4 prints: :hi:mom
# perl5 prints: hi:mom
-=item * BugFix
+=item * B<-e> behavior fixed
Perl 4 would ignore any text which was attached to an B<-e> switch,
always taking the code snippet from the following arg. Additionally, it
# perl4 prints:
# perl5 dies: No code specified for -e.
-=item * Discontinuance
+=item * C<push> returns number of elements in resulting list
In Perl 4 the return value of C<push> was undocumented, but it was
actually the last value being pushed onto the target list. In Perl 5
# perl4 prints: second new
# perl5 prints: 3
-=item * Deprecation
+=item * Some error messages differ
Some error messages will be different.
-=item * Discontinuance
+=item * C<split()> honors subroutine args
In Perl 4, if in list context the delimiters to the first argument of
C<split()> were C<??>, the result would be placed in C<@_> as well as
being returned. Perl 5 has more respect for your subroutine arguments.
-=item * Discontinuance
+=item * Bugs removed
Some bugs may have been inadvertently removed. :-)
=over 4
-=item * Parsing
+=item * Space between . and = triggers syntax error
Note the space between . and =
# perl4 prints: more string
# perl5 prints: syntax error at - line 1, near ". ="
-=item * Parsing
+=item * Better parsing in perl 5
Better parsing in perl 5
# perl4 prints: hello, world
# perl5 prints: syntax error
-=item * Parsing
+=item * Function parsing
"if it looks like a function, it is a function" rule.
# perl4 prints: is zero
# perl5 warns: "Useless use of a constant in void context" if using -w
-=item * Parsing
+=item * String interpolation of C<$#array> differs
String interpolation of the C<$#array> construct differs when braces
are to used around the name.
- @ = (1..3);
+ @a = (1..3);
print "${#a}";
# perl4 prints: 2
# perl4 prints: {a}
# perl5 prints: 2
+=item * Perl guesses on C<map>, C<grep> followed by C<{> if it starts BLOCK or hash ref
+
+When perl sees C<map {> (or C<grep {>), it has to guess whether the C<{>
+starts a BLOCK or a hash reference. If it guesses wrong, it will report
+a syntax error near the C<}> and the missing (or unexpected) comma.
+
+Use unary C<+> before C<{> on a hash reference, and unary C<+> applied
+to the first thing in a BLOCK (after C<{>), for perl to guess right all
+the time. (See L<perlfunc/map>.)
+
=back
=head2 Numerical Traps
=over 5
-=item * Numerical
+=item * Formatted output and significant digits
-Formatted output and significant digits
+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:
- 7.375039999999996141
- 7.37503999999999614
+ 7.3750399999999996141
+ 7.375039999999999614
# Perl5 prints:
7.373504
- 7.37503999999999614
+ 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.
-=item * Numerical
+=item * Auto-increment operator over signed int limit deleted
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
use Math::BigInt;
-=item * Numerical
+=item * Assignment of return values from numeric equality tests doesn't work
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
+Logical tests now return a null, instead of 0
$p = ($test == 1);
print $p,"\n";
=over 5
-=item * (Arrays)
+=item * Negative array subscripts now count from the end of array
Negative array subscripts now count from the end of the array.
# perl4 prints: The third element of the array is 4 also expressed as
# perl5 prints: The third element of the array is 4 also expressed as 4
-=item * (Arrays)
+=item * Setting C<$#array> lower now discards array elements
Setting C<$#array> lower now discards array elements, and makes them
impossible to recover.
# perl4 prints: Before: abcde, After: ab, Recovered: abcd
# perl5 prints: Before: abcde, After: ab, Recovered: ab
-=item * (Hashes)
+=item * Hashes get defined before use
Hashes get defined before use
Perl will now generate a warning when it sees defined(@a) and
defined(%h).
-=item * (Globs)
+=item * Glob assignment from localized variable to variable
glob assignment from variable to variable will fail if the assigned
variable is localized subsequent to the assignment
# perl4 prints: This is Perl 4
# perl5 prints:
-=item * (Globs)
+=item * Assigning C<undef> to glob
Assigning C<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).
+including SEGVs). Perl 5 will also warn if C<undef> is assigned to a
+typeglob. (Note that assigning C<undef> to a typeglob is different
+than calling the C<undef> function on a typeglob (C<undef *foo>), which
+has quite a few effects.
-=item * (Scalar String)
+ $foo = "bar";
+ *foo = undef;
+ print $foo;
+
+ # perl4 prints:
+ # perl4 warns: "Use of uninitialized variable" if using -w
+ # perl5 prints: bar
+ # perl5 warns: "Undefined value assigned to typeglob" if using -w
+
+=item * Changes in unary negation (of strings)
Changes in unary negation (of strings)
This change effects both the return value and what it
# perl4 prints: aab : -0 : 1
# perl5 prints: aab : -aab : aac
-=item * (Constants)
+=item * Modifying of constants prohibited
perl 4 lets you modify constants:
# Modification of a read-only value attempted at foo.pl line 12.
# before: a
-=item * (Scalars)
+=item * C<defined $var> behavior changed
The behavior is slightly different for:
# perl 4: 1
# perl 5: <no output, $x is not called into existence>
-=item * (Variable Suicide)
+=item * Variable Suicide
Variable suicide behavior is more consistent under Perl 5.
Perl5 exhibits the same behavior for hashes and scalars,
=over 5
-=item * (list context)
+=item * Elements of argument lists for formats evaluated in list context
The elements of argument lists for formats are now evaluated in list
context. This means you can interpolate list values now.
# perl4 errors: Please use commas to separate fields in file
# perl5 prints: foo bar baz
-=item * (scalar context)
+=item * C<caller()> returns false value in scalar context if no caller present
The C<caller()> function now returns a false value in a scalar context
if there is no caller. This lets library files determine if they're
# perl4 errors: There is no caller
# perl5 prints: Got a 0
-=item * (scalar context)
+=item * Comma operator in scalar context gives scalar context to args
The comma operator in a scalar context is now guaranteed to give a
scalar context to its arguments.
# Perl4 prints: x = c # Thinks list context interpolates list
# Perl5 prints: x = 3 # Knows scalar uses length of list
-=item * (list, builtin)
+=item * C<sprintf()> prototyped as C<($;@)>
-C<sprintf()> funkiness (array argument converted to scalar array count)
-This test could be added to t/op/sprintf.t
+C<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:
@z = ('%s%s', 'foo', 'bar');
$x = sprintf(@z);
- if ($x eq 'foobar') {print "ok 2\n";} else {print "not ok 2 '$x'\n";}
+ print $x;
- # perl4 prints: ok 2
- # perl5 prints: not ok 2
+ # perl4 prints: foobar
+ # perl5 prints: 3
-C<printf()> works fine, though:
+C<printf()> works the same as it did in Perl 4, though:
+ @z = ('%s%s', 'foo', 'bar');
printf STDOUT (@z);
- print "\n";
# perl4 prints: foobar
# perl5 prints: foobar
-Probably a bug.
-
=back
=head2 Precedence Traps
=over 5
-=item * Precedence
+=item * LHS vs. RHS of any assignment operator
LHS vs. RHS of any assignment operator. LHS is evaluated first
in perl4, second in perl5; this can affect the relationship
# perl4 prints: left
# perl5 prints: right
-=item * Precedence
+=item * Semantic errors introduced due to precedence
These are now semantic errors because of precedence:
# perl4 prints: n is 3, m is 6
# perl5 errors and fails to compile
-=item * Precedence
+=item * Precedence of assignment operators same as the precedence of assignment
The precedence of assignment operators is now the same as the precedence
of assignment. Perl 4 mistakenly gave them the precedence of the associated
now works as a C programmer would expect.
-=item * Precedence
+=item * C<open> requires parentheses around filehandle
open FOO || die;
open(FOO || die);
# perl4 opens or dies
- # perl5 errors: Precedence problem: open FOO should be open(FOO)
+ # perl5 opens FOO, dying only if 'FOO' is false, i.e. never
-=item * Precedence
+=item * C<$:> precedence over C<$::> gone
perl4 gives the special variable, C<$:> precedence, where perl5
treats C<$::> as main C<package>
# perl 4 prints: -:a
# perl 5 prints: x
-=item * Precedence
+=item * Precedence of file test operators documented
perl4 had buggy precedence for the file test operators vis-a-vis
the assignment operators. Thus, although the precedence table
# perl4 prints: no output
# perl5 prints: Can't modify -e in concatenation
-=item * Precedence
+=item * C<keys>, C<each>, C<values> are regular named unary operators
In perl4, keys(), each() and values() were special high-precedence operators
that operated on a single hash, but in perl5, they are regular named unary
=over 5
-=item * Regular Expression
+=item * C<s'$lhs'$rhs'> interpolates on either side
C<s'$lhs'$rhs'> now does no interpolation on either side. It used to
interpolate $lhs but not $rhs. (And still does not match a literal
# perl4 prints: $b 2 $a $b
# perl5 prints: 1 2 $a $b
-=item * Regular Expression
+=item * C<m//g> attaches its state to the searched string
C<m//g> now attaches its state to the searched string rather than the
regular expression. (Once the scope of a block is left for the sub, the
}
sub doit{local($_) = shift; print "Got $_ "}
- # perl4 prints: blah blah blah
+ # perl4 prints: Got blah Got blah Got blah Got blah
# perl5 prints: infinite loop blah...
-=item * Regular Expression
+=item * C<m//o> used within an anonymous sub
Currently, if you use the C<m//o> qualifier on a regular expression
within an anonymous sub, I<all> closures generated from that anonymous
my($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 $right as they were the I<first> time that build_match()
was called, not as they are in the current call.
-This is probably a bug, and may change in future versions of Perl.
-
-=item * Regular Expression
+=item * C<$+> isn't set to whole match
If no parentheses are used in a match, Perl4 sets C<$+> to
the whole match, just like C<$&>. Perl5 does not.
# perl4 prints: bcde
# perl5 prints:
-=item * Regular Expression
+=item * Substitution now returns null string if it fails
substitution now returns the null string if it fails
Also see L<Numerical Traps> for another example of this new feature.
-=item * Regular Expression
+=item * C<s`lhs`rhs`> is now a normal substitution
C<s`lhs`rhs`> (using backticks) is now a normal substitution, with no
backtick expansion
# perl4 prints: <the local hostname>
# perl5 prints: hostname
-=item * Regular Expression
+=item * Stricter parsing of variables in regular expressions
Stricter parsing of variables used in regular expressions
# perl4 prints: foo
# perl5 prints: foobar
-=item * Regular Expression
+=item * C<m?x?> matches only once
Under perl5, C<m?x?> matches only once, like C<?x?>. Under perl4, it matched
repeatedly, like C</x/> or C<m!x!>.
# perl4 prints: perl4
# perl5 prints: perl5
+=item * Failed matches don't reset the match variables
+
+Unlike in Ruby, failed matches in Perl do not reset the match variables
+($1, $2, ..., C<$`>, ...).
=back
=over 5
-=item * (Signals)
+=item * Barewords that used to look like strings look like subroutine calls
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.
$SIG{'TERM'} = SeeYa;
print "SIGTERM is now $SIG{'TERM'}\n";
- # perl4 prints: SIGTERM is main'SeeYa
- # perl5 prints: SIGTERM is now main::1
+ # perl4 prints: SIGTERM is now main'SeeYa
+ # perl5 prints: SIGTERM is now main::1 (and warns "Hasta la vista, baby!")
Use B<-w> to catch this one
-=item * (Sort Subroutine)
+=item * Reverse is no longer allowed as the name of a 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;
+ print sort reverse (2,1,3);
- # perl4 prints: yup yup yup yup abc
- # perl5 prints: abc
+ # perl4 prints: yup yup 123
+ # perl5 prints: 123
+ # perl5 warns (if using -w): Ambiguous call resolved as CORE::reverse()
-=item * warn() won't let you specify a filehandle.
+=item * C<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.
=over 5
-=item * (SysV)
+=item * SysV resets signal handler correctly
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 (HPUX) prints: Got INT...
# perl5 (HPUX) prints: Got INT... Got INT...
-=item * (SysV)
+=item * SysV C<seek()> appends correctly
Under SysV OSes, C<seek()> on a file opened to append C<<< >> >>> now does
the right thing w.r.t. the fopen() manpage. e.g., - When a file is opened
=over 5
-=item * Interpolation
+=item * C<@> always interpolates an array in double-quotish strings
@ now always interpolates an array in double-quotish strings.
# 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
-=item * Interpolation
+=item * Double-quoted strings may no longer end with an unescaped $
-Double-quoted strings may no longer end with an unescaped $ or @.
+Double-quoted strings may no longer end with an unescaped $.
$foo = "foo$";
- $bar = "bar@";
- print "foo is $foo, bar is $bar\n";
+ print "foo is $foo\n";
- # perl4 prints: foo is foo$, bar is bar@
+ # perl4 prints: foo is foo$
# perl5 errors: Final $ should be \$ or $name
Note: perl5 DOES NOT error on the terminating @ in $bar
-=item * Interpolation
+=item * Arbitrary expressions are evaluated inside braces within double quotes
Perl now sometimes evaluates arbitrary expressions inside braces that occur
within double quotes (usually when the opening brace is preceded by C<$>
Note that you can C<use strict;> to ward off such trappiness under perl5.
-=item * Interpolation
+=item * C<$$x> now tries to dereference $x
-The construct "this is $$x" used to interpolate the pid at that
-point, but now apparently tries to dereference $x. C<$$> by itself still
-works fine, however.
+The construct "this is $$x" used to interpolate the pid at that point, but
+now tries to dereference $x. C<$$> by itself still works fine, however.
+ $s = "a reference";
+ $x = *s;
print "this is $$x\n";
# perl4 prints: this is XXXx (XXX is the current pid)
- # perl5 prints: this is
+ # perl5 prints: this is a reference
-=item * Interpolation
+=item * Creation of hashes on the fly with C<eval "EXPR"> requires protection
Creation of hashes on the fly with C<eval "EXPR"> now requires either both
C<$>'s to be protected in the specification of the hash name, or both curlies
# and is compatible for both versions
-=item * Interpolation
+=item * Bugs in earlier perl versions
perl4 programs which unconsciously rely on the bugs in earlier perl versions.
# perl4 prints: This is not perl5
# perl5 prints: This is perl5
-=item * Interpolation
+=item * Array and hash brackets during interpolation
-You also have to be careful about array references.
+You also have to be careful about array and hash brackets during
+interpolation.
+
+ print "$foo["
+
+ perl 4 prints: [
+ perl 5 prints: syntax error
print "$foo{"
perl 4 prints: {
perl 5 prints: syntax error
-=item * Interpolation
+Perl 5 is expecting to find an index or key name following the respective
+brackets, as well as an ending bracket of the appropriate type. In order
+to mimic the behavior of Perl 4, you must escape the bracket like so.
+
+ print "$foo\[";
+ print "$foo\{";
+
+=item * Interpolation of C<\$$foo{bar}>
-Similarly, watch out for:
+Similarly, watch out for: C<\$$foo{bar}>
- $foo = "array";
+ $foo = "baz";
print "\$$foo{bar}\n";
- # perl4 prints: $array{bar}
+ # perl4 prints: $baz{bar}
# perl5 prints: $
-Perl 5 is looking for C<$array{bar}> which doesn't exist, but perl 4 is
-happy just to expand $foo to "array" by itself. Watch out for this
+Perl 5 is looking for C<$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 C<eval>'s.
-=item * Interpolation
+=item * C<qq()> string passed to C<eval> will not find string terminator
C<qq()> string passed to C<eval>
=over 5
-=item * DBM
+=item * Perl5 must have been linked with same dbm/ndbm as the default for C<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
# perl5 prints: ok (IFF linked with -ldbm or -lndbm)
-=item * DBM
+=item * DBM exceeding limit on the key/value size will cause perl5 to exit immediately
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
=item * C<split> on empty string with LIMIT specified
- $string = '';
+ $string = '';
@list = split(/foo/, $string, 2)
Perl4 returns a one element list containing the empty string but Perl5
https://perl5.git.perl.org / perl5.git / blobdiff