...
      elsif ( $added_text =~ /^sample index: (\d+)/ ) {
         $progress_width = 25 + 370 * $1 / $total_samples;
         $progress_canvas->create(
                               'line', '25', '40', $progress_width, '40',
                               -width => '40',
                               -fill => 'blue',
                               -cap => 'butt',
                              )
            if $progress_canvas;
      }
...