There are several ways to go from HTML to XML, so you can use XML tools with it:

• install XML::PYX (and HTML::TreeBuilder) and do pyxhtml file.html | pyxw > file.xml,
• use tidy. Just do tidy --output-xhtml yes file.html > file.xml. Note that you can get a Perl wrapper for it: sl-tidy.pl

Note that if you are only working with HTML it might not be really usefull to convert everything to XML, and you might want to use HTML::Parser instead.