Internally an IV does not need to be mapped to an int, but it might be a long or even long long

Indeed. I'd go further and speculate that an IV is mapped to a long or long long on almost all 64-bit platforms because the two overwhelmingly dominant 64-bit programming models (LP64 and LLP64) both use 32-bit ints. That is, I'd expect to see:

intsize='4';
ivsize='8';
[download]

on just about every 64-bit Perl.

I remember this from a fascinating thread a few months back, where syphilis noted he's "never struck a perl where $Config{intsize} != 4".