Mastering Perl/Tk is more than useful and far more complete than the CPAN documentation. It is good you mentioned it. In fact, it is a part of the reason I know that the CPAN documentation is woefully incomplete.
But like most tutorials, they are primarily concerned with explaining the concepts (how to write a GUI) rather than laying out the range of tools available. Some fairly common use cases are missing: for example, how to create a list box or combo box where the user can add elements to the list. Is this something that has to be created out of lower level components? Is there a core widget with this functionality built in? Part of what I need to do is assess how much custom programming will be needed and how much can rely on existing components. That is hard to do without an overview laying out the capabilities of Tk.
Update Link removed: apparently not legal - JavaFan and private communication. O'Reilly does provide on-line access to this book, but they have changed their format and now require you to subscribe to their Safari On line book service to get access.