How?

To download your own copy of a CPAN mirror, use one of these:

Why?

Why would you want to do this? Because it means you can grep ALL of CPAN, looking for advice on how other people used the same technique that you're figuring out.

Because finding out what other code uses the module that you're beating your head against is a good way to figure it out.

How big?

As of June 2011, it was 1.3gb for the most recent version of all non-developer packages.