Quick help for the problem of never seeing "case 1": you need to tell the program to redraw the widget. One way (possibly not the best, but it works):

sub change {
  $LB->configure(-text=>'case 1');
  $LB->update;
  sleep 3;
  $LB->configure(-text=>'case 2');
}
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As for the problem of doing a longer job in the background while the GUI continues smoothly, I guess you'll have to programm a multitask/multiprocess script. How you do that depends on what exactly you want to do in the background job.

Have fun ...

Andreas