Some indication of what you have tried, documentation you don't understand, etc. would be helpful.

Here is CPAN's XML::LibXML Documentation. I suggest looking at the Document, Node and Element modules first; then other modules as necessary (e.g. cross-referenced from another module).

In addition, here's W3C's XPath Specification which you should also find useful. I believe there may be a Perl interface to this; I don't have details (as I've only ever used XPath directly in this situation).

In my opinion, your learning curve in this area would be best served by having a go at writing this from the documentation then asking more specific questions if you run into difficulties.

I'll be happy to provide further assistance with any problems you run into.