thanks to everyone! I should also note here that Rob and Ken Williams from the Inline mailing list suggested this:

<Rob:>To use gdb, I gather that perl, itself, needs to have been compiled with '-g'. There's a helpful discussion of debugging XS in "Extending and Embedding Perl" by Jenness and Cozens (published by Manning).

Also 'sv_dump()' and the Devel::Peek module's 'Dump()' function allow you to see what's inside your perl structures - which can be a useful debugging aid (depending on the nature of the problem).

<Ken:>Yes, that's true, I've done it. It then becomes a standard exercise in debugging a C program using gdb - the fact that it's reading perl code and executing it is immaterial (and largely inaccessible) from the perspective of gdb.