I picked up the O'Reilly book "Learning Perl/Tk" out of a book bin at a local book store. It cost me all of $7.00. While I haven't actually used a GUI on any of my Perl projects, I've enjoyed going through the book and learning about Perl/Tk. I usually pull it out when I'm feeling like I need a change in perspective. It's just different enough from what I'm usually working on that it provides me with some amusement.

While I wouldn't recommend spending big bucks on a book about a technique you may never need, if you can pick up something inexpensive, it's worth the investment. For myself, if in the future I find I need a GUI for something, I'll be partly prepared to tackle that.