I'm just learning Perl::Tk, but I'm guessing that you will need to do something like trap all keyboard and mouse events on the main window, and minimize on any event. You should probably also watch for the main window losing focus, which could indicate a system keystroke like SAS (Secure Attention Sequence) or the "Windows" key. If you can bear to read VB, Microsoft has an example of the API calls.
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Spring: Forces, Coiled Again!