Oh I realize that, but the thing is the hashes seem to get created okay, its just during printing that the machine hits a memory problem. And, deleting keys as I print them solves the issue: that's the puzzling part. Where in my nested-while loops would memory be getting used? The printing gets about half-way through before crashing, so at that stage I can't see why additional RAM would get used (it's 32-bit perl, so while the box has more maximum addressable would be 4Gb I guess).

Update: Also, it occurred to me that there should only be (5001 * 5000 / 2) = ~12.5 million elements in each hash, so not quite so bad as having three hashes of 25 million!