That will never work. It will run the machine out of memory.

It will attempt to build 4 huge lists:

1. a list of integers, one for every record in the file.

   Input to the first map.

2. a list of anonymous arrays -- each containing two elements -- one for every record in the file.

   Output from the map, input to sort.

3. Same again, but reordered.

   Output from sort, input to second map.

4. An ordered list of all the records.

   Output from second map, input to for.

If the OPs records average 64 bytes, that would require:

• 4 * 20GB = 80GB
• + 6 * 335544320 * 24 bytes (for the scalars) = 45GB
• + 335544320 * 232 bytes (for the anonymous arrays ) = 72.5GB
• + lots of other bits and pieces of overhead within Tie::File and Perl's internals stacks = ??

For a grand total memory requirement of at least 197.5 Gigabytes