Make dual-lived constant.pm work on 5.8 again

Make dual-lived constant.pm work on 5.8 again

Before releasing the version of constant.pm from bleadperl to the CPAN,
I tested it with the versions of Perl I have by hand, and it appears
that the current code fails to compile on 5.8:

  Bareword "_DOWNGRADE" not allowed while "strict subs" in use at
  lib/constant.pm line 142.

Added by bd8cb5529605f33aa9cf95d6c471386b3a0e015d

Removing the short-circuit return allows the code to compile and the
tests to pass on all stable Perl from 5.8.2 to 5.16.1.