perlfaq3 offers this advice concerning using less memory.

How can I make my Perl program take less memory?

In some cases, using substr() or vec() to simulate arrays can be highl
+y beneficial. For example, an array of a thousand booleans will take 
+at least 20,000 bytes of space, but it can be turned into one 125-byt
+e bit vector--a considerable memory savings. The standard Tie::Substr
+Hash module can also help for certain types of data structure. If you
+'re working with specialist data structures (matrices, for instance) 
+modules that implement these in C may use less memory than equivalent
+ Perl modules.
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I did some playing around with Tie::SubstrHash a while ago, but my results were inconclusive for my purposes, but depending upon your data, it might prove more frugal than using a standard hash.