I thought the CPAN server is just an FTP server, isn't it? I would just poke around inside the CPAN mirror and do some reverse engineering :-D

CPAN.html                               1k Nov 19  2001 Symbolic link -> authors/id/J/JO/JONO/cpan.html
ENDINGS                                 1k May  1  2001 Symbolic link -> .cpan/ENDINGS
MIRRORED.BY                           123k Dec 16 11:42
MIRRORING.FROM                          3k Jan 11 18:23
README                                  1k May 26  2002
README.html                             1k May  1  2001 Symbolic link -> index.html
RECENT                                  1k May  1  2001 Symbolic link -> indices/RECENT-print
RECENT.html                           175k Jan 11 03:34
ROADMAP                                 1k May 26  2002
ROADMAP.html                            1k May 26  2002
SITES                                  21k Dec 16 11:42
SITES.html                             42k Dec 16 11:42
authors/                                7k Feb 17  2003 Directory
clpa/                                   1k Aug 30  2002 Directory
disclaimer.html                         2k Aug 30  2002
doc/                                    1k Sep  8 20:34 Directory
index.html                              3k Jan 10 22:01
indices/                                1k Jan 11 18:26 Directory
jhi/                                    1k Feb 18  2003 Directory
ls-lR.gz                                1k May  1  2001 Symbolic link -> indices/ls-lR.gz
misc/                                   1k Dec 27  2002 Directory

modules/                                1k Oct  1 11:27 Directory          <<< looks interesting

ports/                                  1k Jan  3 20:35 Directory
scripts/                                1k May  9  2002 Directory
src/                                    1k Nov 17 15:45 Directory

The CPAN.pm module provides a standard interface to CPAN. There is a Defaultsite variable which can be overrided with your own FTP/CPAN server... I haven't tried this but it would certainly be an interesting little weekend project.

use CPAN;
$CPAN::Defaultsite = "ftp://mirror.aarnet.edu.au/pub/CPAN";
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