I like this idea but I'm having trouble get a 2 pass solution to work nicely. In the first pass I identify which line numbers need to be removed. But then I'll need to mung that list into some convenient form to use while reading the file again in the second pass...

Tie::File looks quite handy, but since I don't want to alter the original log file, I'd have to copy it first and then reduce it, which could be a problem if disk-space is tight.

So, so far storing the whole file in a hash so that I can properly delete the lines no longer required seems like a winner.

Salva: The 'sort' idea has the problem that a given lock ID can be locked and unlocked multiple times in the file, so the sorted valued won't always be 'locking' follwed by 'unlocked'. Plus the contents of the logfile need to remain in the correct order for analysis of the remaining contents... but thanks!