Thank you for asm tips, but I have a C API that sometimes wants SSE2 data as arguments in a C function prototype. I am not trying to use SSE2 cpu instructions or intrinsics directly. If I were, I would know exactly what part of the SSE2 union the data should be in.

Maybe I shouldn't be reinventing the wheel and instead looking for a CPAN module that already understands what a C union is and somehow integrate it into my code and just say "supply a Foo::CAggregate object ref for parameter 1" :-/