Computer programs that write computer programs

I've been following the author's writing and publishing saga through his mailing list for over a year, now, but Higher-Order Perl is about to be released. He promises a complete copy of the book free on his site, too.

What makes this such an interesting book is that it is entirely based on the concept of writing Perl code to generate other Perl code. Your scripts could conceivably re-write themselves using these techniques. I makes my head hurt sometimes, but I look forward to trying this out...