Probably the key improvement here is the use of hexadecimal notation: 0xffffffff instead of the corresponding, “magic” decimal. As long as you are very careful to use unsigned arithmetic and specify bitmasks that are no larger than the integer-size you know that you are using, code like this ought to be transportable. (It might not be readable, heh, but it ought to be int-length agnostic.)

I would almost edit my comment to say, don’t use “arithmetic” at all when you are bit-twiddling, as in code like this:

$num = $num - 4294967296 if $num > 4294967295;
$num = $num + 4294967296 if $num < -2147483648;

That’s nasty... And I recognize at least one of those numbers, -2147483648, as an old, familiar 32-bit friend. I also detect the presence of signed arithmetic, dependent upon the thirty-first bit being the sign-bit. I suggest that this code should be rewritten to use bit-masking operators ... and to do so, perhaps, in a very specific way, as follows:

If you want to mash-off, say, all but the rightmost 31 bits of an integer quantity, you should take: qty := qty and ( not 0x7fffffff );. (I am not using Perl notation here, for clarity.) The subexpression, (not 0x7fffffff), will evaluate to an int in which all bits are 1 except for the rightmost 31 bits, and it will do so correctly no matter what the sizeof(int) may be.

So, I suggest that you need to go through code like this (fortunately, it is a small piece of code), work out what it is doing and then rewrite it. Construct a very thorough set of Test::More cases that you can run on both a 32-bit and a 64-bit system to prove that your rewrite is thoroughly correct.