Well, there's nothing to stop you doing this in Perl (read up on 'stat'), but a cron might be simpler (using 'find').
place something like:
find '/usr/home/foobar/' -name '*.add' -mtime +7 -exec rm {} \;
in your crontab.
BTW I don't use *nix much, so don't take this as gospel, more of a very IMHO.
Tom Melly, tom@tomandlu.co.uk