Agreed, premature optimizing is a bad thing.

Also in the not recommended category is trying to write your own profiler by peppering your code with calls to Time::HiRes::time. Much better to run the code through a proper profiler such as Devel::NYTProf, that will do a much better job, will profile everything including the stuff you did not think was a bottleneck, and will give you a nicely formatted report to look at.