jc23,
Even though this really isn't a Perl question:

From the man page for Sendmail on my HPUX 11.0 machine:
.forward files are described in the aliases(5) manpage.

From section 5 of the man pages for aliases on my HPUX 11.0 machine:
If a file named .forward exists in a user's home directory and is owned by the user, sendmail redirects mail for that user to the list of addresses in the .forward file.

An address in a .forward or :include: file can be anything that can appear as a mailing-list in the alias text file.

Hope this helps - L~R