Fix spurious dependency bumps, bring back to 5.6

Note - while I tested things extensively, it is not easy to test 5.6 currently,
as a lot of the depchain was subject to bitrot. I currently have pending fixes
for File::Spec, Capture::Tiny, File::Temp and File::Path, each containing some
trivial nit preventing otherwise 5.6-compliant code from working

Patches will be sent upstream in due time