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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
# 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
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- * 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
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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|
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Note that you can use strict; to ward off such trappiness under perl5.
- * Interpolation
-
The construct "this is $$x" used to interpolate the pid at that point, but now
tries to dereference $x. $$ 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
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- * 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($foobar{'baz'})) ? (print "Yup") : (print "Nope");
# perl4 prints: Yup
# perl5 prints: Nope
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Changing
eval "\$$hashname{'$key'} = q|$value|";
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to
eval "\$\$hashname{'$key'} = q|$value|";
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causes the following result:
# perl4 prints: Nope
# perl5 prints: Yup
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or, changing to
eval "\$$hashname\{'$key'\} = q|$value|";
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causes the following result:
# perl4 prints: Yup
# perl5 prints: Yup
# and is compatible for both versions
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- * 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
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- * Interpolation
-
You also have to be careful about array references.
print "$foo{"
perl 4 prints: {
perl 5 prints: syntax error
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- * Interpolation
-
Similarly, watch out for:
$foo = "baz";
print "\$$foo{bar}\n";
# perl4 prints: $baz{bar}
# perl5 prints: $
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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.
- * Interpolation
-
qq() string passed to eval
eval qq(
foreach \$y (keys %\$x\) {
\$count++;
}
);
# perl4 runs this ok
# perl5 prints: Can't find string terminator ")"
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