in reply to Finding a LCS module on word level
See http://en.wikipedia.org/wiki/Longest_common_substring_problem for a dynamic programming algorithm. You will find a perl implementation in this Wikibooks thing. It is char based, but you can easily make it word based by splitting /\s+/ instead of // (don't forget to change $m and $n as well).
(This is basically what ikegami already said.)
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