Line-based reading is not, in general, very efficient from an IO standpoint. It is buffered IO, but it typically only reads in about 64K blocks. You should see performance improvement by using something like:

while (sysread (MYFILE, $myBuffer, 10*1024*1024) > 0) {
  # process buffer line by line
}
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regardless of whether you thread or not. My guess is that using 2 threads in this manner would be more efficient, but only if you're on a dual-processor system (I'm not even sure a hyper-threaded/dual-core processor would help).