I'm not sure if it's still the case (I recall some discussion about optimising it) but using the more obvious[1] idiom of foreach my $line (<$FH>) { ... }

I'm not sure if it's still the case (I recall some discussion about optimising it) but using the more obvious[1] idiom of foreach my $line (<$FH>) { ... } slurps the entire file because the foreach is forcing list context on the diamond operator. The while(<$FH>) { ... } that you describe works as desired though. http://stackoverflow.com/questions/585341/ http://stackoverflow.com/questions/585341/ appears to cover it in some detail. [1] To me, anyway.