I think an even more common reason is a bit of a bug in CPAN. The problem occurs when the newest version of a module on CPAN is contained in the full Perl distribution.

Some authors of modules that are included in the Perl distribution try to make a new version after (or just before) a major release of Perl. That way CPAN will fetch the "newer" (but identical other than version number) release of the module instead of asking you if you want to install all of the standard Perl distribution in order to get one module.

        - tye (but my friends call me "Tye")