According to the Google README doc, the deserialization does not occur in SOAP::Lite:

It doesn't really matter where the deserialization actually occurs...  Google/AdWords/Deserializer.pm loads SOAP::Lite and tries to set up an inheritance relationship with SOAP::Deserializer. And that's the line where the error occurs...

FYI I tried your BEGIN { push @ISA, "SOAP::Deserializer"; } fix and got the following error ...

Sorry, my fault. That should've been BEGIN { our @ISA = ... (as I've updated my node in the meatime).