Ah, figured there was something that I either missed or didn't understand. I've never really tried looking under the hood to understand fully what Perl is doing behind the scenes. I didn't realize that the foreach command would make a copy of the data structure.

Now that you've enlightened me about this, I realize how foolish my idea was. And that leads me back to thinking along the lines of what you had said in the post that I responded too. (By the way, thanks for teaching me something new. I sincerely appreciate you kindly pointing out something that I overlooked.)

   My question is how to spew all of the keys into a file without making an intermediate structure that contains all of the keys.

Well, here's a thought on that. I'm assuming that there had to have been some Perl code that created the keys in the hash. If you have access to modify that code, modify it so that it's printing to a file instead creating the hash. That leaves the unsorted keys in a file and no initial hash. That in turn frees up more memory for a sorting method, which can be applied as the keys are written to file and/or after all keys have been written to file.

In other words, "extract" the keys before the hash is created and then do the sort. Then after the sorting is complete, create the hash.

Of course, if the code where the hash keys are added cannot be modified for some reason, the above idea can't be implemented.

Does that sound like a reasonable idea or have I missed something else due to my lack of knowledge and experience?