$canvas->createRectangle(
             $i * $ps, $j * $ps,
             $i * $ps + $ps, $j * $ps + $ps,
             -fill    => $c,
             -outline => $c,
             -tags    => ['bg']);
      }

##</code><code>##

sub erase{
    $canvas->delete('bg');
}

##</code><code>##

       $canvas->createLine($start_x, $start_y, $x1, $y1,
                           -width => 2,
                           -fill  => 'white',
                           -tags  => ['fg']);

# ...
# and after drawing the new background:

   $canvas->raise('fg', 'bg');