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;
    }
}