if ($counter =~ /00$/) {
can be written as if ($counter % 100 == 0) {
which saves perl from converting the number to a string and checking it against a regexp.
Comment on Re^2: Live operations-per-second counter
(But then it depends on how much one is familiar with such a construct - for me it is quite as intuitive as the former, but it's obvious that others' mileage may vary)