It tells perl you're not going to change the regex, so its only compiled once, not every time, (ie its no good if you're using variables in your regex).

next if($_ =~ m/^#/); try as next if($_ =~ m/^#/o);

Execute your script with time to benchmark the performance, ie time parser.pl. If you get better performance, cool, you've got it for almost no effort, if you dont, well, you've spent almost no time on it..

While its not exactly IO related, it may speed up your program.