Perhaps you could test the non $^S branch by embedding the code under test in a separate litttle script and arrange to have this code executed in a separate process executed using an IPC::Run::<something> call and/or system(). You could then capture any error output/exit code as needed to verify correct behavior?
Yes, I did try something like this, and it worked, but Devel::Cover did not detect it. So the branch is not really uncoverable, I originally wrote, but rather "uncoverable as far as Devel::Cover can tell." :-)
