Building a Vector Space Search Engine in Perl (O'Reilly)
As a Perl programmer, sooner or later you'll get an opportunity to build a search engine. Like many programming tasks - parsing a date, validating an e-mail address, writing to a temporary file - this turns out to be easy to do, but hard to get right. Most people try end up with some kind of reverse index, a data structure that associates words with lists of documents. Onto this, they graft a scheme for ranking the results by relevance."