To solve your immediate problem:

$txt -> delete('0.0', 'end'); 
  if ($todo){   
    $txt -> insert('end',"Let's see what you picked...\n\n\n");
    $txt->update;
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However, you need to stop using sleep in a gui program. Why? Because what if you have other things going on in the background, in a more complex script? sleep will stop them all from functioning for 2 seconds. sleep puts the WHOLE script to sleep. Use this:

#  sleep (2);
$mw->after(2000);    #milliseconds
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