You might want to take a look to brian_d_foy's Building My Own CPAN.

The presentation explains how to...

• use CPAN-Mini to get a copy of the latest modules available on CPAN (currently less than 900MB),
• filter out some dists or modules (by name or auhtor),
• inject other dists or modules not available on CPAN by using CPAN::Mini::Inject,
• and even include obsolete stuff from BackPAN.

However, the quick answer to your question maybe is in slides 23 and 34:

CPAN::Shell->o(
   qw( conf urllist unshift ),
   $where_i_found_cpan,
);
[download]

Where $where_i_found_cpan (e.g. file:///MINICPAN) is a directory containing the typical CPAN structure with:

• authors/id/A/AB/ABC/...
• modules/...

HTH

Update: Added an example for $where_i_found_cpan using the file:///... prefix.