$mw->idletasks();

##</code><code>##

$canvas->idletasks();

##</code><code>##

for ... (...) {
    ...
    if (...) {
        $canvas->idletasks();
    }
}

$canvas->idletasks();

##</code><code>##

   for my $i (0..$size-1){
      for my $j (0..$size-1){
         my $a = ($i%20) > 9;
         my $b = ($j%20) > 9;
         my $c;
         if($a^$b){
            $c = sprintf("#%02X0000",$i%0xFF);
         } else {
            $c = sprintf("#00%02X00",$j%0xFF);
         }
         $USE[$i][$j]=$c;
      }
   }

##</code><code>##

   for my $i (0..$size-1){
      my $i_col = $i%0xFF;
      my $i_mod20_9 = $i%20>9;
      for my $j (0..$size-1){
         $USE[$i][$j] = $i_mod20_9^($j%20>9)
                      ? sprintf("#%02X0000",$i_col)
                      : sprintf("#00%02X00",$j%0xFF);
      }
   }

##</code><code>##

   for my $i (0..$size-1){
      my $i_col = $i%0xFF;
      my $i_mod20_9 = $i%20>9;
      for my $j (0..$size-1){
         $USE[$i][$j] = $i_mod20_9^($j%20>9) ? $reds[$i_col] : $greens[$j%0xFF];
      }
   }

##</code><code>##

if (exists $use_cols{$size}) {
    @USE = @{$use_cols{$size}};
}
else {
    for ... {
        ...
        $USE[$i][$j] = ...;
    }

    $use_cols{$size} = [@USE];
}