With toplevel widgets, i believe you can set the options so the widget will allways be in focus. One of the options for top level widgets is -takefocus or -focusmodel.
To make sure check the Perl/TK documentation. If you are working on Win, you might also want to look into trying Win32::GUI.
Thanks for the suggestion. $window->focusmodel("active") doesn't seem to work (under NT at least). -takefocus is related to tab-key traversal of widgets.
Looks like another one of those platform-dependent things that Tk hasn't got round to. Win32::GUI is something I'm looking into just now.