notification about desktop switch

I am relatively new to perl tk - love it, but cant solve the following

My perl/tk application (Mandriva 2010.0) uses a popup help feature.
When I hover over a widget, a toplevel widget is created and will
display the help text, and I delete it when the focus leaves the
popped up window -no problem so far.

I call the overrideredirect method on the help toplevel widget to drop
the window decoration - and here my problem comes: I am using KDE4,
and can switch between desktops. Due to the overrideredirect, the
popup window is "sticky" and appears on each desktop. How can I make
the popup window be deleted when a desktop switch occurs? I am
familiar with using events and binding procedures to events - would be
nice to have an event that is launched when a desktop switch occurs -
but I have not found any such event.

An alternative solution would be to find a way to un-decorate the
window without the drawback that the windowmanager does not handle
overrideredirect windows.

Any advice please?