The simplest way to do this is to search the entire master directory, and iterate over the record set.

use strict;
use Net::LDAP;

my $ld = Net::LDAP->new('myserver');
$ld->bind(dn=>'username',password=>'passwd');

my $result = $ld->search(base=>'cn=mydir',
                         scope=>'sub',
                         filter=>'(objectclass=*)');

while (my $entry = $result->entry())
{
  #do comparison here
}
[download]

You'll probably need to use page support and/or async searching if your directory contains much in the way of data.

One thing to watch out for: I've done this sort of thing a few times, and tended to find that detecting when entries/attributes aren't there is the trickiest bit. It's also worth noting that there are commercial apps available that will do this sort of thing for you.