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.

Unfortunately, it's not always quite as simple in the general case.  For example, if the 32-bit code relies on specific overflow/wrap-over effects. Consider the following very simple case:

my $result = (0xffffffff + 2) & 0xffffffff;
print $result;
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Although the result is masked down to 32 bits, it differs depending on the perl being used:

32-bit perl: 4294967295
64-bit perl: 1
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(I'm not saying it cannot be done, but it requires taking into account the specific overflow behavior, IV/UV to NV conversions of 32-bit perls, etc. — simple 0xffffffff AND-masking isn't enough.)