1. Create a "master" Thread (usually the root thread).
2. Create some (possibly configurable) number of child threads
3. (Here's the tricky part). You've got a couple of alternatives:
   • a) on thread->create(), pass the fileno() to each child thread, along with an offset and skip count. Each child thread then does an open(INF, '<&', $fileno) on the fileno, reads/discards skip count lines, then iteratively read/process/ skip until EOF
   • b) alternately, create 2 Thread::Queues (one from master to children, the other from children to master). Master reads each line from file and posts to downstream queue; children grab (randomly) a line off the queue, process it, then post a response to the upstream queue.

As ever, TIMTOWTDI. (b) is probably simpler, but (a) is more deterministic. Both can be mimiced using a process based approach.

I've successfully used (a) for ETL tools, but if your file is binary/random access, it can get complicated to skip records. Also, if the children are writing to an output file, (b) might be easier to let a single master thread do the writing instead of coordinating writes betwen children.