$insertId = $dbh->{'mysql_insertid'}

     ...

     mysql_insertid
         MySQL has the ability to choose unique key values
         automatically. If this happened, the new ID will be
         stored in this attribute. This attribute is not valid
         for DBD::mSQL. An alternative way for accessing this
         attribute is via $dbh->{'mysql_insertid'}. (Note we are
         using the $dbh in this case!)