The typemap is a mapping from C types to Perl internal types. There are many predefined types, but as you can imagine, it would be impossible to list them all. It looks like double ** isn't in the typemap (which doesn't quite surprise me). So, you have to either extend the typemap, or use the appropriate Perl internal types.

I don't know how to extend the typemap. My knowledge of XS is extremely limited.

Abigail