If you didn't know, "backpan" is all the PAUSE uploads but with no deletions (PAUSE, by Andreas König, is the Perl Authors Upload SErver which is the way the vast majority of content enters CPAN now).
We could use more though! It doesn't have to be public, but it'd be nice to know that we had a few more backups of old old CPAN distributions.
First mirror CPAN if you aren't already. Once a day is fine. Also once a day then run something like the following:
rsync --exclude CHECKSUMS -vrptgx ~/mirror/CPAN/authors/id/ ~/mirror/backpan/authors/id/
to copy new files into backpan. Notice there's no --delete argument to that rsync command.