File::Which

Add File::Which back in:

I believe the problem was that the perl in the path was being used in various places rather than the install perl, so in the first commit I replaced perl with $^X, even if we don't use File::Which, I think this is the right thing to do.

I tested this by installing on a perlbrew built perl but had /usr/bin/perl in the path first.

I think that requires is the appropriate section for File::Which; I believe that build_requires is more for modules that aren't needed after install, and is primarily useful for building rpms/debs that don't include build only deps.