If you have total control over the method of adding an entry to your MySQL database, why not add a procedure to that method that adds an entry to your OpenLDAP database at the same time? That way you only have to select the entire set of records from the MySQL database once, solving your data concurrency issues with pretty low overhead. If you were to use the DBI for connecting to both databases you could even use the same SQL statements to insert data into both places. (I'm using the term "database" loosely, LDAP is technically a directory service).

If you're really worried about someone adding entries to the MySQL database and bypassing the corresponding OpenLDAP data addition, you can easily run a script on a regular basis to verify that the all of the data is synchronized.

As far as modules for connecting to the two databases, the DBI is the obvious choice for the MySQL database. Perl-LDAP is a nice module for doing LDAP interation, and I also like DBD::LDAP.