yep, so if the indices in your arrays are arbitrary or if your multidimensional array is sparse (like a sparse matrix) you'd better use hashes of hashes.
for instance if you declare $a->[10000] = 1 it also autovivivies elements 0 until 9999 setting the value to undef. This takes up unnessessary (? difficult word;) space, and makes loops much more cumputation intensive.

Cheers,
Rob