It's a great book. It's basically about using Perl as a functional programming language. He starts off with a general discussion of recursion, from classic problems like Tower of Hanoi to writing a basic parser. He talks about some of the problems that come up (like slowness due to fucntions being called over and over with the same arguments) and the solutions (caching, easily implemented with Memoize.pm, the inner workings of which are discussed). By the end, he is building a vector graphics drawing language.

Generally well written, and FWIW (quite a lot actually), Mark is a nice and helpful guy if you contact him with any issues or whatever. Well worth the bucks, you will still be looking at this years from now.