I think that BackPAN is very new and it hasn't been officially announced yet. I saw some mentions of it in Use Perl journals but not elsewhere. Here is some information that I received in an email when I deleted files:

    Note: To encourage deletions (and keep CPAN CDROMable), there is a
    project underway to maintain the complete PAUSE history on a site 
+of
    its own, most probably history.perl.org. A preliminary version is
    available at ftp://pause.perl.org/pub/backpan/authors/id/
[download]

--
John.