I'd probably do what others have suggested, fetch the rows separately and combine them in Perl. But you might be interested to know that MySQL can do this all for you:

SELECT CONCAT("Line ", line_n, "    ", 
              GROUP_CONCAT(word ORDER BY word_position SEPARATOR " "))
FROM words 
GROUP BY line_n
ORDER BY line_n;
[download]

GROUP_CONCAT is a powerful tool indeed - very useful in report building.

-sam