sub count_up {
{
    .
    .
    .
}
}

##</code><code>##

$cnt=0; $progress=0
$cnt=0; $progress=20
$cnt=0; $progress=40
$cnt=0; $progress=60

##</code><code>##

my $r = \$progress;
print("address of \$progress=$r\n");

##</code><code>##

$cnt=0; $progress=0
address of $progress=SCALAR(0x19fc970)
$cnt=0; $progress=20
address of $progress=SCALAR(0x19fe710)
$cnt=0; $progress=40
address of $progress=SCALAR(0x1cfdb0c)
$cnt=0; $progress=60
address of $progress=SCALAR(0x1d13fc4)

##</code><code>##

    $pb->configure(
        -from => 0,
        -to   => 20,
        -variable => \$progress,  ## store the reference.
    );
$progress = 0;

##</code><code>##

$cnt=0; $progress=0
$cnt=0; $progress=0
$cnt=0; $progress=0
$cnt=0; $progress=0

##</code><code>##

sub variable {
    my $c = shift;
    my $oldvarref = $c->{'-variable'};
    my $oldval = $$oldvarref if $oldvarref;
    if(@_) {
	my $varref = shift;
        if ($oldvarref)
         {
	  $c->traceVdelete($oldvarref);
         }
	$c->{'-variable'} = $varref;
	$c->traceVariable($varref, 'w', sub { $c->value($_[1]) });
	$$varref = $oldval;
	_layoutRequest($c,2);
    }
    $oldval;
}

##</code><code>##

use strict;
use warnings;

use Tk;
use Tk::ProgressBar;

my $mw = MainWindow->new(-title=>"Bug Demo");

my $progress;
my $pb = $mw->ProgressBar(
    -width    => 20,
    -length   => 200,
    -blocks   => 20,
    -from     => 0,
    -to       => 20,
    -variable => \$progress,  ## store the reference.
)->pack();

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

MainLoop;


sub count_up {
    $progress = 0;
    $mw->update;
    for (1..20) {
        select(undef, undef, undef, 0.01);
        $progress++;
        $mw->update;
    }
}