use Tk ();
use Tk::Dialog ();

my $mw = MainWindow->new();
$mw->protocol('WM_DELETE_WINDOW', \&ExitApplication);
# ------------------------------------------------------------
sub ExitApplication {
    my $dialog = $mw->Dialog(-text => 'Programm wirklich beenden?',
                             -bitmap => 'question',
                             -title => 'Programm beenden',
                             -default_button => 'Yes',
                             -buttons => [qw/Yes No/],
                             );

    my $answer = $dialog->Show(); # and display dialog
    if (lc($answer) eq 'yes'){
        # perhaps save data if necessary...
        exit;
    }
    else {
        # continue
    } # else

} # ExitApplication