You could try packing your numbers into a 64-bit int; it might save some space:

 ++$hash{ pack 'Q', $n_1to20 * 1.2e6**2 + $a_1to1_2e6 * 1.2e6 + $b_1to
+1_2e6 };

[download]

It depends on the mix of sizes of the larger numbers. (I'll think on it some more.)

Also, try pre-extending your hash to 3 million: keys %hash = 3e6;

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In reply to Re^5: Tallying co-occurence of numbers by BrowserUk
in thread Tallying co-occurence of numbers by K_Edw

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