To be honest, I didn't use the book this time. I had a dim recollection of "screenheight" so I grepped the Tk directory and found Wm.pm and Wm.pod.
If you're looking at buying the book, I got mine with the Perl CD Bookshelf v3 - includes paper Perl in a nutshell and digital "Programming Perl" (3rd edition), "Perl Cookbook", "Mastering Perl/Tk", "Perl and LWP", and "Perl and XML", all for about the price of two books. I'd check ebay.
The fourth version is available now, but it drops the LWP, Tk and XML books in favor of merlyn's latest book and "Mastering Regular Expressions". It also has a newer version of the cookbook.
Update: A quick search on ebay shows several copies for under $20 US, including shipping.