That's a pretty open-ended question.

If you're interested in comparing files, I'd suggest taking a look at Text::Diff; it will compare 2 files the way the Linux/Unix utility diff does.

If you're interested in how, exactly, the algorithms for comparison work, take a look at Approximate_string_matching.  It references the Levenshtein_distance, which is a way of calculating how close a set of strings are to one another.

Furthermore, if you Google for "Levenshtein distance", you'll find sites that provide more detailed analysis of the algorithms, including some, like this one, which present a helpful visual demonstration.