This may not be what you want to hear but:

How do I match XML, HTML, or other nasty, ugly things with a regex?

Do not use regexes. Use a module and forget about the regular expressions. ... XML::LibXML ...

Other XML modules often recommended are XML::Twig, XML::Rules, or XML::Compile. Personally I prefer XML::LibXML or XML::Twig. (You may run into XML::Simple too, but that is only appropriate for really simple cases, of which this does not appear to be one, so I'd avoid it.)

If you could show some representative sample input and the corresponding expected output, someone might be able to get you started with one of those modules.