Not specific to Perl, but tips for getting the lay of the XML land:

Altova offers a free "Home Edition" of XMLSpy, which can be a very handy visual aid for those learning about XML, what it looks like, how to edit it, how to get started with "offshoots" like XML Schema, XSL/XSLT, etc.

refcards.com has a few reference cards on XML and related technologies. (I learned about that site from Perlreref as formatted quick reference.)

Zvon has many similar materials, freely downloadable, comprehensive, in various formats and numerous languages.

As far as learning about new modules and/or code snippets, I have found the bundled documentation helpful ;-). But don't forget about utilities such as POD2HTML... Other helpful creatures in this beastiary include podgen, DoxyFilt* (Doxygen for Perl), and Perl::Tidy.

Some books I have found useful are: XSL Essentials, XSLT, XSLT Cookbook, and of course, The Perl Cookbook, 2nd Ed.

HTH,

Update: See also Perl-XML Frequently Asked Questions and Frequently Asked Questions about XML::Simple

* Update: 2005-12-28 Kudos to both john_oshea and tfrayner for alerting me to the fact that my link above has been rendered usless by the foul creatures known as spammers... I have found what appears to be a good link to obtain DoxyFilt; the most recent version seems to be from August 24, 2005: Doxygen-0.84.tar.gz. Thanks again, guys!

* Update: 2006-03-11 Like Zvon, deepX has a good collection of "Quick References" on XML-related technologies such as XPath, XSD, XSL and XSLT (and even CSS and MySQL!).