There is no magic to sprinkle over an algorithm that relies on 32-bit integers to make it behave with other integer sizes. The hardcoded numbers here are magic for 32-bit numbers:

    $num = $num - 4294967296 if $num > 4294967295;
    $num = $num + 4294967296 if $num < -2147483648;
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You will have to understand the algorithm involved to be able to make the required changes. Alternatively, you can run the program under a Perl compiled with 32-bit integers.