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;
  ...
}
[download]

into a subroutine like this:

sub handler {
    ...
    my $fname = $q->param('fname');
    my $lname = $q->param('lname');
    ...
    sub foo {
      my $name = $fname.$lname;
      ...
    }
}
[download]

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
[download]

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');
...
[download]

   MeowChow                                   
               s aamecha.s a..a\u$&owag.print