Pressing any of the buttons on a DialogBox causes it to close, the callback you give to -command is called when you press any button, and is given the name of the pressed button as its argument. So, one answer is that you already have a callback sub that gets called.

The second is that calling Show returns the text of the pressed button, when the DialogBox is closed, so you can also just do:

  my $answer = $config->Show();
  myApplySub() if($answer eq 'Apply');
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C.