I've worked with Overture's XML interfaces before and they're pretty straightforward XML Request / Response kind of architecture. The documentation they provided was fairly adequate, but like any tech documentation, it can take a bit of messing around before you can figure out exactly how it all works.

If you haven't played with XML before, then look at some CPAN modules for parsing it - my favourite is XML::LibXML.

There are also numerous articles / books / tutorials available, but probably you won't need these. The docs from Overture and a few test scripts should be enough.