Update perl_builder.rb

Setting cpan_mirror only if the build is enabled.
Forcing users to use your "default_cpan_mirror" unless they set environment cpan_mirror = "" seems counter intuitive.
From cpanm documentation:
Defaults to http://search.cpan.org/CPAN which is a geo location aware redirector.
Merge pull request #13 from johncm/master

CPAN_MIRROR issues. -- Good fix, thank you for contribution John!