What I really want to do is to work with a third-party DLL for which I have a dll.h header file and an Exports.def file. I don't have a static version

I don't know how you can create a static library with that, but you could create an import library for the dll:

dlltool --kill-at --input-def your.def --output-lib lib_choose_a_name.
+a
[download]

Then you need to link to lib_choose_a_name.a when building the project.
I don't know how to link the XS code to that import library without using ExtUtils::MakeMaker.

The dll will still be need to be found at runtime (so it needs to be in the path or the cwd) but the code in the XS file you originally presented should then need no modification - assuming, of course, that "dll.h" contains the declaration of the "own_function" function (and that your dll exports that function).

Cheers,
Rob