...if you would like to install version 1.23_90, you need to know where the distribution file resides on CPAN relative to the authors/id/ directory. If the author is BAR, this might be BAR/Foo-1.23_90.tar.gz; so you would have to say

          install BAR/Foo-1.23_90.tar.gz

       The first example will be driven by an object of the class CPAN::Module, the second by an object of class CPAN::Distribution.