Thank you for recognizing that your lack of description is leading us into providing inadequate solutions. You also have to recognize that changing requirements (5 million to 30 million is a significant difference), will also lead to us wasting time and energy.

You still have done an inadequate job of describing all of the requirements in order for us to provide a solution that meets your requirements. Each time a monk provides a new solution based on your "it didn't work because X", you reply with "that didn't work because Y". In order for us to help, you need to define all the parameters of the problem first. I mentioned a number of things in Re^3: Dear Monks. Since your current issue seems to be memory, consider a few more: What operating system? How much physical memory? Is perl 32 or 64 bit?

There is a relatively simple solution if order doesn't matter but since you haven't removed that as a constraint for us - it is rather difficult to guess what will satisfy your unwritten requirements.

Additionally, you have to understand that PerlMonks is not a free script writing service. We expect you to show effort. If you don't know where the perl documentation is - please see Perldoc online though a local copy was probably installed and available from the command line. With that said, I would be happy to provide a solution to you once you do a better job at describing the requirements but I am not going to keep guessing with "try this".