I've had to do this once before, and never could find a good way to handle it. So I cheated, and kept a nextUID() function, that connected to an Oracle database, and use a sequence that I had set up there.

But I already had the Oracle database, and my LDAP routines already opened LDAP connections (as all new users authenticated out of an Oracle database that tracked staff and students)

Of course, this is more an LDAP than Perl question, so I checked the OpenLDAP mailing list, and they had a solution for doing something similar, but storing the number in your LDAP directory:

In reply to Re: Looking for highest uidNumber in ldap by jhourcle
in thread Looking for highest uidNumber in ldap by rlb3

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