Well, if similarity gets called often with the same arguments then Memoize could help, caching values to prevent recalculation...

Another thing to look into is Devel::SmallProf which shows you how many times a line is run and how long your program spends running it, to help you find slowdowns.

You may also find a speedup if you can manage to create $Measure->new($wn) only once instead of at each call, but I'm not sure if the module will allow that, not being familiar with it.