Hello, thanks for the suggestions!

I think I should have been specific -- when I said "not sparse", I actually meant that the data structure had been fully created in advance with a loop exactly like this. I've added in the checks, and before the program the program crashes (about 3 hours of CPU) it does not every autovivify.

One thing that goes on inside the #do great deeds is to change values in the Array part of the HoHoA, and I had expected that to be the cause of my memory leak. I'm really confuzzled as to why this:

foreach my $value ( @$subarray ) {
[download]

would eat up memory. I would have guessed it would use one scalar's worth (perhaps 15 bytes of data), and that this memory would get reused over & over again. Instead, the script runs on this loop for a few hours before using up all the memory.