This works fine, but I have to add security headers which are not defined in the WSDL - you can see a solution at the FAQ but I can't work out how to implement it. Has anyone managed to use this example?

:D the author, obviously :P See XML::Compile::SOAP::WSS

But I don't want to hard code the URL as I feel that defeats the purpose of the WSDL - what if it changes down the line?

Things change, life goes on, adapt/improvise/overcome...

Is there a way to retrieve that namespace URL from the WSDL so I can add it dynamically?

Sure, its just xml, but see next part

Or some other way to get SOAP::Lite to add it properly?

It depends heavily on the part you're leaving out, the part most folks like me know little about, even SOAP::Lite, the WSDL

See https://metacpan.org/pod/SOAP::Lite#BUGS-AND-LIMITATIONS