First you have to optimize for correctness, then you can optimize for speed. At the moment I'm working on a project where I do a lot of serialization, and to ensure that I could debug the correctness I have chosen an easy readable serialization format: YAML.
But running Devel::NYTProf on my code showed that an awful lot of time was spend in the YAML code. Changing a few lines to use Storable instead showed an improvement in speed going from 1000s to 200s for my test data. This is a significant win even while developing, but of course I shouldn't have used the old YAML-implementation to begin with.
Mentioning this to the local Perl Mongers group I got referred to a benchmark of different serialization modules by Christian Hansen (I think). I've updated the script and the interesting part is that JSON::XS outruns Storable on both tests.
Thank you Devel::NYTProf for showing me that 80% of my run time was spent in old pure perl serialization code.