Could you produce and post a small sample program that exhibits this behaviour?

From looking at the source, I believe that $ldap->{net_ldap_mesg} does not exist/is not filled/is undef. Whether this makes sense when you call $ldap->sync(), I don't know, but it would help in debugging the error. Maybe you need to call some other method before you can call sync()? For example if you have no outstanding messages?