sanyacomua

Можно попробовать анализировать buffer:

buffer - scalar reference to object's input buffer
$ref = $obj->buffer;
This method returns a scalar reference to the input buffer for $obj. Data in the input buffer is data that has been read from the remote side but has yet to be read by the user. Modifications to the input buffer are returned by a subsequent read.

Если не пустой - значит после промта еще есть данные и надо ждать второго промта.