The following version:

#!/usr/bin/perl
use strict;
use warnings;
use Tk;
use Tk::ProgressBar;

my $percent_done=0;
my $mw = MainWindow->new(-title => 'Processing');

my $Progress = $mw->ProgressBar(
                 -width => 30,
                -from => 0,
                -to => 100,
                -blocks => 50,
                -colors => [
                        0, 'green',
                        50, 'yellow' ,
                        80, 'red',
                       ],
                -variable => \$percent_done
                   )->pack(-fill => 'x');

$mw->Button(-text => 'Go!', -command=> sub {
          my $i;
          for ($i =0; $i < 1000; $i++) {
        $percent_done = $i/10;
        print STDOUT "$i\n";
        $mw->update; 
          }
        })->pack(-side => 'bottom');

MainLoop();
[download]

The way I made it to work, though, was by using "strict" and "warnings" ...

Cheers,
alf