I do exactly this, but I use the mail system to do it (i.e. you don't need a perl script). I have found qmail to be perfectly suited to this task. Qmail will allow any user to control an entire domain or even several domains. I use a script called qmtppull that gets mail from a remote mailbox and injects it into the local mail system, where it gets delivered normally.

This isn't really a perl answer, but sometimes there are better ways to do it and I think that this is one of them. I brought this up because I was going to try doing this job with perl, but found (in my case anyway) a better way.