I would say that the key fact is that the _Inline subdirectory isn't being created - if there was a naming clash at the root of the problem, I might expect the directory to be created and then later code not to be able to find it, but that's just a wild guess.

As the C code doesn't change, and the error message indicates that the missing subroutine is being looked for in the Device::WH1091 namespace, would it be possible to point the environment variable PERL_INLINE_DIRECTORY at one of the _Inline directories that were built earlier?

It may be possible that your problem prevents the compiled C being created, but will be able to use it if it already exists and can be found.