XML::Parser is pre-installed with Activestate Perl (it is used by ppm), so even if it's not core, it comes pre-installed on the one platform where installation can be a problem. Note that XML::Twig, which doesn't come with Activestate perl is not too hard to install, as you could just drop the Twig.pm file in the module XML directory. Of course it is a pretty big file these days at nearly 400K. I think that XML::Simple is also a single file, so that is an other option. Oh, and remember that XML::Parser needs expat installed (once again installed with Activestate Perl, and easy enough to install on most other systems)

There are also nearly-complete XML parsers in pure perl: XML::SAX::PurePerl and XML::Parser::Lite, which comes with SOAP::Lite. XML::SAX::PurePerl requires you to then install some SAX modules to do anything interesting with it, but XML::Parser::Lite emulates quite well XML::Parser, so you could ship it with your application and use it as a fallback if XML::Parser is not available. It doesn't parse DTDs, which may or may not be a problem.