Come on in, the water's lovely! (really, it's not that difficult)

One thing to note would be after one adds a pp_def, one needs to re-run perl Makefile.PL because the "multi-C" feature bakes the available function names into the generated Makefile so that only things actually changed need rebuilding. A way to "cheat" here is to copy-paste similar functions and just rename them, so the recreating of Makefiles only needs doing once.

I'd suggest calling this function eval2d. You'd need 2D versions of init, new_spline, etc.