If your Tk program is busy doing stuff when you switch to and from another window, then the Tk image will not be re-drawn.

So, as anon suggested, pepper your code with $mw->update() in long loops, or processing sections etc.

For efficiency sake (i.e. don't slow down your program) do NOT

   foreach my $i (1..10000) {
      ... do stuff
      $mw->update();
   }
[download]

instead, only update sometimes (every 100 iterations perhaps?)

   
   foreach my $i (1..10000) {
      ... do stuff
      if (int($i/100)*100 == $i) {
         $mw->update();
      }
   }
[download]