$ok = $obj->cmd($string);
    $ok = $obj->cmd(String   => $string,
                    [Output  => $ref,]
                    [Cmd_remove_mode => $mode,]
                    [Errmode => $mode,]
                    [Input_record_separator => $chars,]
                    [Ors     => $chars,]
                    [Output_record_separator => $chars,]
                    [Prompt  => $match,]
                    [Rs      => $chars,]
                    [Timeout => $secs,]);