sub on_Window_delete_event {

    ...

    my ( $foo, ... ) = ...;

    if ($foo) {

        my $MessageDialog = Gtk3::MessageDialog->new(
            $Window,
            'destroy-with-parent',
            'GTK_MESSAGE_QUESTION',
            'GTK_BUTTONS_YES_NO',
            "Sie haben %d nicht gespeicherte Änderungen, Fenster dennoch schießen", ...,
        );
        my $Response = $MessageDialog->run;
        $MessageDialog->destroy;

        given ($Response) {
            when ("yes") { return Gtk3::EVENT_PROPAGATE; }
            when ("no")  { return Gtk3::EVENT_STOP; }
            default      { ; }
        }
    }
    else {
         return Gtk3::EVENT_PROPAGATE;
    }
}

sub on_Window_destroy {
    Gtk3->main_quit;
}