I cannot claim that the beauty of this interface was immediately obvious to me. My quick read of the interface sees tight but well-commented code. I'm hesitant to dive further in, as I'm already lots of review behind. The part that I think is most worthy of comment, right now, is that it's an interface. This differs from previous standards for code beauty (the author lists Duff's Device and rsync's checksum as examples) because it's not an algorithm, and because it's longer than what would fit on the back of an envelope. Five pages in the book, mostly comments. But the beauty of Perl that I see every day is that I can abstract, for example, the process of downloading a web page from a series of network pokes to something close to
get($url) ;. If the foundations are solid, you can build on them. But nobody normally looks and says "My, what a great foundation."