I apologize for not having enough time at the moment to sift through the code to spot the error. If an answer hasn't shown up in a few hours I might be able to later (update: A few hours later, still not finding time / motivation for sifting through guts/xs. Try #xs on irc.perl.org).

However, I do feel that looking in the right place is important. To that end, it's worth mentioning that it is virtually impossible for this to be an Inline::C issue. Inline::C and Inline really are just automation layers. It's a matter of how your code is dealing with perl(guts|xs|call|api), not of how Inline is automating the XS build process.