perltrap
Current Perl documentation can be found at
perldoc.perl.org.
Here is our local, out-dated (pre-5.6) version:
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 . The third biggest trap is not reading the list of changes in this version
of Perl; see the perldelta manpage.
Accustomed awk users should take special note of the following:
-
The English module, loaded via
use English;
allows you to refer to special variables (like $/ ) with names (like
$RS ), as though they were in awk; see the perlvar manpage for details.
-
Semicolons are required after all simple statements in Perl (except at the
end of a block). Newline is not a statement delimiter.
-
Curly brackets are required on
if s and while s.
-
Variables begin with ``$'', ``@'' 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 the perlvar manpage.
-
$<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 next, exit, and continue keywords work differently.
-
The following variables work differently:
Awk Perl
ARGC $#ARGV or scalar @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 programmers should take note of the following:
Seasoned sed programmers should take note of the following:
Sharp shell programmers should take note of the following:
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 the perldata manpage 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()). (User-defined subroutines can be
only list operators, never unary ones.) See the perlop manpage.
-
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 perlfunc: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 the perlform manpage 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:
- 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 <wjm@best.com> for inclusion. Also note that at least some of these can be caught
with -w.
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, because 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
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 $: .
- 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-element list
sub sub2{ return(1,2,3)} # return a 3-element list
@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: Bareword 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 test.pl 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)){
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 $_ .)
- Discontinuance
split with no arguments now behaves like perlfunc: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
- BugFix
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.
- Discontinuance
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.
@x = ('existing');
print push(@x, 'first new', 'second new');
# perl4 prints: second new
# perl5 prints: 3
- Discontinuance
In Perl 4 (and versions of Perl 5 before 5.004), '\r' characters in Perl code were silently allowed, although they could cause
(mysterious!) failures in certain constructs, particularly here documents.
Now,
'\r' characters cause an immediate fatal error. (Note: In this example, the
notation \015 represents the incorrect line ending. Depending upon your text viewer, it
will look different.)
print "foo";\015
print "bar";
# perl4 prints: foobar
# perl5.003 prints: foobar
# perl5.004 dies: Illegal character \015 (carriage return)
See the perldiag manpage for full details.
- Deprecation
Some error messages will be different.
- Discontinuance
Some bugs may have been inadvertently removed. :-)
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
- Parsing
String interpolation of the $#array construct differs when braces are to used around the name.
@ = (1..3);
print "${#a}";
# perl4 prints: 2
# perl5 fails with syntax error
@ = (1..3);
print "$#{a}";
# perl4 prints: {a}
# perl5 prints: 2
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 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;
- 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 General Regular Expression Traps using s///, etc.
for another example of this new feature...
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:
- (Globs)
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).
- (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: $_[0]\n";
}
# perl4:
# before: x after: m
# before: a after: m
# before: m after: m
# before: m after: m
# Perl5:
# before: x after: m
# Modification of a read-only value attempted at foo.pl line 12.
# before: a
- (Scalars)
The behavior is slightly different for:
print "$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 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
- (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=
@<<<<< @||||| @>>>>>
@fmt;
.
write;
# 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.
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.
- Precedence
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.
@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 parentheses around the filehandle. Otherwise,
perl5 leaves the statement as its 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
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 $foo .= "q" should parse as
perlman:perlop, it actually parses as (-e ($foo .= "q")) . In perl5, the precedence is as documented.
-e $foo .= "q"
# perl4 prints: no output
# perl5 prints: Can't modify -e in concatenation
- Precedence
In perl4,
keys(),
each() and
values() were special high-precedence operators that operated on a single hash, but in perl5, they are regular named unary operators. As documented, named unary operators have lower precedence than the arithmetic and concatenation operators
+ - . , but the perl4 variants of these operators actually bind tighter than + - . . Thus, for:
%foo = 1..10;
print keys %foo - 1
# perl4 prints: 4
# perl5 prints: Type of arg 1 to keys must be hash (not subtraction)
The perl4 behavior was probably more useful, if less consistent.
All types of
RE traps.
- Regular Expression
s'$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 $_ "}
# perl4 prints: blah blah blah
# perl5 prints: infinite loop blah...
- Regular Expression
Currently, if you use the m//o qualifier on a regular expression within an anonymous sub, all closures generated from that anonymous sub will use the regular expression
as it was compiled when it was used the very first time in any such
closure. For instance, if you say
sub build_match {
my($left,$right) = @_;
return sub { $_[0] =~ /$left stuff $right/o; };
}
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.
This is probably a bug, and may change in future versions of Perl.
- 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
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
- (SysV)
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.
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 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.
open(TEST,">>seek.test");
$start = tell TEST ;
foreach(1 .. 9){
print TEST "$_ ";
}
$end = tell TEST ;
seek(TEST,$start,0);
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
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: someone@somewhere.com\n";
# perl4 prints: To:someone@somewhere.com
# perl5 errors : In string, @somewhere now must be written as \@somewhere
- 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 perlfunc:eval if possible.
$hashname = "foobar";
$key = "baz";
$value = 1234;
eval "\$$hashname{'$key'} = q|$value|";
(defined($foobar{'baz'})) ? (print "Yup") : (print "Nope");
# perl4 prints: Yup
# perl5 prints: Nope
Changing
eval "\$$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
- 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
perlman:perlop string passed to eval
eval qq(
foreach \$y (keys %\$x\) {
\$count++;
}
);
# perl4 runs this ok
# perl5 prints: Can't find string terminator ")"
General
DBM traps.
- DBM
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)
- DBM
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:
sub foo {
$rc = do "./do.pl";
return 8;
}
print &foo, "\n";
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
Same behavior if you replace do with require.
- 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.
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