It's time to bump the required version of Perl again, to enable use of
subroutine signatures without having to disable warnings everywhere, as
I start integrating what makes sense in make_it_mojo back into master.

This takes inspiration from the techniques used by the Modern::Perl
module (which enables warnings, strict flag, and minimum Perl version
controls). Now future Perl version upgrades, or changes to standard
features and experiments, can all be done from one spot.

It is deliberate to avoid doing this in kdesrc-run, so that script can
remain self-contained.  I will look at the test cases later.