Updated version of library/perl to bundle cpanm

The tagged version of docker-perl includes cpanm on top of the Perl 5 core.  I'm not sure if it makes sense to do that or not, but my gut feeling, for cpanm, was to go for it.  I'm happy to hear arguments on either side.

I also used a date+version tag instead of the SHA for clarity's sake, tested with brew-cli.