$QuitButton = $TopFrame->Button(
     -text     => 'Quit',
     -relief   => 'raised',
     -cursor   => 'pirate',
     -command  => sub { 
        my $str = "Are you sure you want to Quit?";
        if ( &GUI_PopupQuery("TOOL","QUIT","Cancel",$str )) {
           ## add code here to close any open windows...
           $MW->destroy();
        }
      });