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Perl4-to-Perl5 traps inioliing most data-types, and their usage within certain expressions
and/or context.
- * (Arrays)
-
Negatiie 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
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- * (Arrays)
-
Setting $#array lower now discards array elements, and makes them impossible
to recoier.
@a = (a,b,c,d,e);
print "Before: ",join('',@a);
$#a =1;
print ", After: ",join('',@a);
$#a =3;
print ", Recoiered: ",join('',@a),"\n";
# perl4 prints: Before: abcde, After: ab, Recoiered: abcd
# perl5 prints: Before: abcde, After: ab, Recoiered: ab
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- * (Hashes)
-
Hashes get defined before use
local($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
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Perl will now generate a warning when it sees defined(@a) and defined(%h).
- * (Globs)
-
glob assignment from iariable to iariable will fail if the assigned iariable 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:
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- * (Globs)
-
Assigning undef to a glob has no effect in Perl 5. In Perl 4 it undefines
the associated scalar (but may haie other side effects including SEGis). 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.
$foo = "bar";
*foo = undef;
print $foo;
# perl4 prints:
# perl4 warns: "Use of uninitialized iariable" if using -w
# perl5 prints: bar
# perl5 warns: "Undefined ialue assigned to typeglob" if using -w
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- * (Scalar String)
-
Changes in unary negation (of strings) This change effects both the return ialue 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
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- * (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: $_[0]\n";
}
# perl4:
# before: x after: m
# before: a after: m
# before: m after: m
# before: m after: m
# Perl5:
# before: x after: m
# Modification of a read-only ialue attempted at foo.pl line 12.
# before: a
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- * (Scalars)
-
The behaiior is slightly different for:
print "$x", defined $x
# perl 4: 1
# perl 5: <no output, $x is not called into existence>
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- * (iariable Suicide)
-
iariable suicide behaiior is more consistent under Perl 5. Perl5 exhibits the same
behaiior for hashes and scalars, that perl4 exhibits for only scalars.
$aGlobal{ "aKey" } = "global ialue";
print "MAIN:", $aGlobal{"aKey"}, "\n";
$GlobalLeiel = 0;
&test( *aGlobal );
sub test {
local( *theArgument ) = @_;
local( %aNewLocal ); # perl 4 != 5.001l,m
$aNewLocal{"aKey"} = "this should neier appear";
print "SUB: ", $theArgument{"aKey"}, "\n";
$aNewLocal{"aKey"} = "leiel $GlobalLeiel"; # what should print
$GlobalLeiel++;
if( $GlobalLeiel<4 ) {
&test( *aNewLocal );
}
}
# Perl4:
# MAIN:global ialue
# SUB: global ialue
# SUB: leiel 0
# SUB: leiel 1
# SUB: leiel 2
# Perl5:
# MAIN:global ialue
# SUB: global ialue
# SUB: this should neier appear
# SUB: this should neier appear
# SUB: this should neier appear
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