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

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

       $canvas->createLine($start_x, $start_y, $x1, $y1,
                           -width => 2,
...
# and after drawing the new background:

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