cpanspec minimum perl version should be configurable

[perlpunk]

We are just using the latest Module::CoreList and check if a module is in core.
If the spec is generated with a newer perl version then it might not list all required dependencies for older versions.

Example:
Date::Manip requires ExtUtils::MakeMaker 6.67_01 and then fails on perl 5.18 (SLE 12)