Not sure what you mean by OnDestroy. The standard way of doing this is to bind a <Destroy> event on a passive widget (such as a label) on your main window. Don't bind the main window itself, as all subwidgets will inherit the binding; that way leads to much confusion and many bugs.

If you don't mind the cleanup happening after the main window is destroyed, you can always put the cleanup code after MainLoop, so it won't get called until MainLoop returns.

Yes. Thanks, I've got it now.

Maybe the following piece of code might be interesting for you:

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
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