I'm guessing that I'm not using the same version of Perl that the .so was built for but admittedly that's a guess.

Yep, compiled XS modules need to be run by the same version as perl (edit: that they were compiled with). Maybe you need only the same major version of perl?

Anyone have an idea how I would proceed ?

Ask the vendor which version the .so is compiled for, have them compile it for your version of perl, or try different versions of perl by trial and error. Time to build your own perl instead of using the bundled perl.

Looks like you probably need perl 5.9. I googled "Perl_Tstack_sp_ptr" and this seems to be the clearest record of breaking compiled perl modules by upgrading to a new major version of perl: https://bugs.archlinux.org/task/9078