How are you requesting the attributes from Perl? modifyTimestamp is a "operational attribute" and won't be returned unless you either explicitely ask for it, or request the special "+" attribute, which will return operational attributes. If you want all attributes, you'll need + and *. For example:
Code:
ldapsearch -LLL -x -D uid=zimbra,cn=admins,cn=zimbra -w zimbra '(objectclass=*)' + '*'
The man page on ldapsearch explains this behavior.