Take another look. The last line of the loop, $buffer = substr($buffer, rindex($buffer, "\n")); in conjunction with the length $buffer as the fourth parameter to sysread has the effect of grabbing any partial line from the end of the buffer and moving it to the beginning where the next buffer load is then appended.

In this way, every line is processed as a complete line whilst benefiting from reading large chunks from the file without resorting to slurping the whle file into memory.

See my post below for two variations of the algorithm and a benchmark.