the DialogBox actually poped up, but it is not the top window

Usually, a

  $window->raise;
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raises your window, but in the case of a dialog, you might need to do something like:

  $mw->after(50, sub {$dialog->raise});
  $dialog->Show;
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to schedule a callback to be executed after the dialog is shown. Untested :)

Also, if you don't need your MainWindow, why don't you withdraw it?