Every data structure in Perl, whether it's a scalar, an array or a hash takes some memory overhead (in addition to the memory needed to store the data itself). For a scalar, this is about 24 bytes. For hashes and arrays, this is even 2 or 3 times more. And note that if you have an array with 10 elements, you pay overhead for one array and 10 scalars.

In Perl. it's a lot cheaper to have 2 arrays with 10000 elements each, than to have 10000 arrays with 2 elements each. So, if you have lots and lots of data, you should give it some thought on how you are going to organize it. Arrays take less memory than hashes, but I it's probably more worthwhile to reduce the number of aggregates than to replace hashes with pseudo-hashes (which are dead anyway).

You also might want to look into the Devel::Size CPAN module.

Abigail