Since I have avoided perl+setuid like the plague I cannot really comment on a perl solution for you. No doubt there is more than one! You mention that maintenance might become a problem with a greater number of users, so you may well discover that a clever script now, will give rise to some more clever scripts, followed by some desperate ones - over time. It may well be the case that rethinking the mail-server strategy will save you work, pain, stress and mileage in the long run.(YMMV)

You mention LDAP++ , I have found LDAP a fantastic way to retrieve user information, particularly with perl, much software supports LDAP authentication also. I am drifting OT now , but will recommend you check out some of the features of InsightServer (v4.0.0b which I am re-evaluating ATM) and DBMail, who specifically mention..

Security.
     Dbmail has got nothing to do with the filesystem or interaction w
+ith other programs in the Unix
     environment which need special permissions.
     Dbmail is as secure as the database it's based upon.
[download]

Those are my two cents, most of this I am still researching. Insightserver is _ALMOST_ a MSExchange killer but it does cost $$ (less than Exchange), it still makes a nice db-backed imap server, and from what I see today the v4 beta has had it's administration interface VASTLY upgraded. I discover that Vacation is part of its web admin tools.