No, don't uninstall XML::SAX, it provides the base class used by all the SAX parsers, so it's a prerequisite for XML::SAX::Expat.

In the beginning, there was only XML::Parser. It provides a good fast parser (via libexpat) but it's API isn't very friendly. Then the SAX API was created to serve as a common API. This means that you can plug in different parsers without changing your code. The last SAX parser you install will be the system default from that point on. The Perl XML FAQ has links to Kip Hampton's introductory articles.