Hmm, your requirement is not very clear to me (and probably to other monks as well, judging from the answers you've got so far), but if you want to compare lists, it seems to me that the List::Util and List::MoreUtils CPAN modules might be the first place to go.
Comment on Re: Clustering/classifying recommendations
Essentially think of each input line represents a point in an N-dimensional space. I want to classify/cluster these points and need a metric to measure the separation/distance.
one point= (category 1, category 2, category 3…. category N)