Whoops - me bad, there is a better way in Tk - use OnDestroy. Now that I'm back at work I've looked at some of the Tk stuff that I've done and this is what I've used.

$main_window->OnDestroy(sub {
                kill 15 => $kid;
                exit;
                }
            );
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The OnDestroy is a Tk routine, should do the trick. Get rid of the myexit sub and use this, you should be Ok. kill does work - just make sure you are killing the right pid :-)