Well, there is not much I can contribute... because in my eyes, optimization is best done by using better algorithmns and datastructures...

Before optimisation, it might help quite a bit finding what to optimize. Maybe the Module Devel::DProf might help you finding which subs are called how often and how much time it did. Then you could decide if there's a better algorithm, or if it's worth implementing this sub in C or the like or even using external programs which perform better for a certain problem. Or it is possible to run Tasks parallel (maybe on a multi-cpu-machine)?

$reverseString = reverse($string);
instead of:
$reverseString = join("", reverse split(//, $string);
[download]

Try to use good sorting algorithms, e.g. the Schwartzian Transform (or Guttman Rosler Transform) or the Orcish Maneuvre.
Try not to use tie's.
Try caching data
Try eq instead of =~ /^...$/
Try not to use $&, $` and $´

My basic feelings about optimization are: buy a faster machine, because it often is cheaper than optimisation :-)

Best regards,
perl -le "s==*F=e=>y~\*martinF~stronat~=>s~[^\w]~~g=>chop,print"