To introduce the subject, the best is to try the program Drum Patterns with its default values. If you are already a drummer, it will remind you something. For the others I'll try to explain its useful purpose. This little program constitute, I think, a interesting demonstration of what can be done with Perl, CGI.pm and the HTTP protocol.
The software archive can be downloaded with ftp on the site linux-france.com.
I start with an analogy with the written language. We write texts which are made up of paragraphes. Paragraphs are made up of sentences, sentences are made up of words and words are made up of letters. When someone wants to become a drummer, he has to work lots of exercises, stodgy at first sight, that help learning words, the vocabulary. The analogy is limited because the letters (the alphabet) are only a few. These letters are the strokes done by both hands and feet, right and left. The vocabulary is not predefined, it is "free". The goal is to generate all the possible words from an alphabet and some work rules, proved by experience.