hi butchie,
Unfortunately, SOAP::WSDL's XML serializer doesn't give you fine grained control over namespaces right now, so the only way is to write your own serializer class. That's not really difficult - you'll need to write something similar to SOAP::WSDL::Serializer::XSD and put the relevant information into the envelope.
Then, all you need to do is to register your new serializer class with SOAP::WSDL::Factory::Serializer (see the man page on how to do that).

Martin