Namespace URIs don't really point anywhere. They are simply a mean to create non-conflicting namespaces for tags and attributes in XML documents. If you happen to own domain mydomain.com and you design your own DTD you may for example use http://mydomain.com/mydtd
URI for tags and attributes you use. It is unluckely to conflict with DTDs created by somebody else as they will have URIs with other domain part and you can always choose non-conflicting URIs in your own domain for other your DTDs.
Read Namespaces in XML for more info.
Update: You post reminded me to publish somewhere my patches for XML::LibXML which make xpath queries to documents with namespaces easier. I've just posted it to perl-xml mailing list, you may find it useful.
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