mysql 插入前 锁表问题

$dbh = DBI->connect("dbi:mysql:database=$db_name;host=$ip;port=3306",$user,$passwd,{
                          RaiseError => 1,
                          AutoCommit => 0
                        }) or die "can't connect to database ". DBI-errstr;
     eval{
    $dbh->do("select count(*) from t1 for update") ;
    $dbh->do("insert into t1 values(87)") ;
     $dbh->commit;
        };
    if( $@ ) {
    #warn "Database error: $DBI::errstr\n";
    $dbh->rollback(); #just die if rollback is failing 
    	 };
  $dbh->disconnect;