What's the best XML parser for Perl?
I think you are using a pretty good one. XML::LibXML, Matt Sergeant and Christian Glahn's Perl interface to Daniel Velliard's libxml2 is one of the faster XML Parsers that I know of.
It really depends on your needs, as people have said. To parse XML files that were ~100Mb in size (gene annotations from TAIR, 1 file per chromosome), I used mirod's XML::Twig module, which lets you set callbacks to parse the elements that interest you, presenting each sub-document as an XML::Simple tree. It combines the benefits of a SAX parser (scanning the file as a stream) with a DOM parser (working more easily with the interesting pieces).
If you need speed, power or features, XML::LibXML is the way to go. If you're after ease of use, though, XML::Simple is a viable alternative.