The specifications for RSS 0.91, 1.0 and 2.0 are easy to read and understand, especially if you are not responsible for implementing the small details. Indeed, Dave Winer's specifications (RSS 0.9x and 2.0) are designed to be as easy to read and implement as possible, at the risk of being looser and less specific.
RSS 0.91: backend.userland.com/rss091
RSS 1.0: web.resource.org/rss/1.0/spec
RSS 2.0: blogs.law.harvard.edu/tech/rss
For some added perspective on RSS, you should look at Dan Bricklin's “about RSS” (and related files) at rss.softwaregarden.com/aboutrss.html and Tim Bray's introduction to RDF at www.xml.com/pub/a/2001/01/24/rdf.html.
The W3C's introduction to RDF is at www.w3.org/RDF.
Finally, you can download XML::RSS from CPAN, the Perl archive network, at www.cpan.org.