I don't see any obvious reason why only 2327 lines would be printed out of 2400. Is it possible you don't really have 2400 distinct key entries in your hash?

As you may be aware, hashes handle key collision by overwriting the existing hash entry, so that if I had two lines with key 2104, the second value to be added to the hash under that key would overwrite the first value.

I don't think you have given us enough to identify the problem. If you really just want to slow things down, add a sleep inside your foreach.