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The bug recently discussed in Unusual Closure Behaviour is considered something of an oddity, with little practical value, but here's what I believe is a valid and defensible use of it, specifically with regards to mod_perl, and more generally with regards to any tool that wraps scripts into callable subroutines/hooks.

Those of you who've developed for mod_perl know that it's a bad idea to create global lexical variables in your Apache::Registry scripts, since they do not stay shared when mod_perl wraps your code into a subroutine. For example, under mod_perl, the following script is transformed from:
#!/usr/bin/perl -w ... my $fname = $q->param('fname'); my $lname = $q->param('lname'); ... sub foo { my $name = $fname.$lname; ... }
into a subroutine like this:
sub handler { ... my $fname = $q->param('fname'); my $lname = $q->param('lname'); ... sub foo { my $name = $fname.$lname; ... } }
As you see, foo() becomes one of those pesky inner named subroutines, which means that upon successive invocations of the script (or more precisely, the handler subroutine), the $fname and $lname variables of the inner sub will remain bound to their values from the first invocation, and will not reflect new values. This issue is well documented in the mod_perl guide, along with several remedies for the problem, none of which I like too much.

Looking over that discussion, it occured to me that the 'my $x if 0' construct can resolve this issue:

sub OUTER { my $x if 0; $x = 0; print "OUTER: \$x is now: ", ++$x, $/; sub INNER { print "INNER: \$x is now: ", ++$x, $/; } } OUTER;INNER;INNER; OUTER;INNER;INNER; ## results OUTER: $x is now: 0 INNER: $x is now: 1 INNER: $x is now: 2 OUTER: $x is now: 0 INNER: $x is now: 1 INNER: $x is now: 2
What's happening here is that successive invocations simply overwrite the values bound upon the initial invocation. Since we've cheated Perl out of enacting the run-time effect of my $x, this allows lexical variables to stay shared between outer and inner named subroutines (despite -w's admonitions to the contrary).

I don't think I would do something like this in production code. I'll probably stick to using package globals, perhaps with our-scoping, but it's certainly worth thinking about and perhaps incorporating as a bonified language feature in future perl releases, eg:

... my $fname : static = $q->param('fname'); my $lname : static = $q->param('lname'); ...
               s aamecha.s a..a\u$&owag.print

In reply to It *is* a feature: mod_perl and 'my $x if 0' by MeowChow

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