I ran across some interesting behavior with lexically-scoped variables; it first struck me as so odd that I thought I'd discovered a bug in my stack¹. However, I couldn't believe that I was doing something so unusual that I was the first to discover such a bug, so I built a smaller test case. What results is not, I think, a bug in the stack (oh, yes it is, sort of: see this reply), but is still behavior I don't entirely understand. Perhaps some more enlightened monk can assist me.

I know, of course, that when I store a reference to a lexically-scoped value, then let the original go out of scope, the reference still exists, so the variable is not entirely destroyed. However, it surprised me to learn that I can enter that scope again, specifically set the value of the variable, and not have it work. Check it out:

#!/usr/bin/perl
package main;
use strict; use warnings;

use Tk;
use Tk::ProgressBar;

my $mw = MainWindow->new(-title=>"Bug Demo");
my $pb = $mw->ProgressBar(
    -width => 20,
    -length => 200,
    -blocks => 20,
)->pack();

my $button = $mw->Button(
    -text => 'Press this!',
    -command => \&count_up,
)->pack();

MainLoop;

sub count_up {
    $mw->update;
    my $progress = 0;
    $pb->configure(
        -from => 0,
        -to   => 20,
        -variable => \$progress,  ## store the reference.
    );
    
    my $cnt = 0;
    print "\$cnt=$cnt; \$progress=$progress\n";
    
    for (1..20) {
        $progress++;
        $cnt++ unless ($_ % 2);
        $mw->update;
    }
    
    $mw->update;
}
[download]

Note how I specifically state my $progress = 0; in the sub. If I click the button 3 times, thus calling the sub 3 times, I get the following console output:

$cnt=0; $progress=0
$cnt=0; $progress=20
$cnt=0; $progress=40
[download]

I don't understand why that's happening -- if I'm setting $progress to zero each time, why does it always increment in that way? I've managed to refactor the production code that has this issue in such a way as to avoid this behavior, so this isn't life-or-death by any stretch. However, it is really bugging me, and I'd appreciate some enlightenment.

Thanks in advance!

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1: i.e. in some module I was using, its dependencies, or perl itself (yes, I know how unlikely the last is).

Updates:

• 20070913 : added link to solution, thanks to ikegami