Perl has three internal numeric types, which vary based on how Perl was compiled.

• IV - an "integer value" which can be 32-bit or 64-bit. It can optionally be 64-bit on 32-bit systems (for a small performance hit) but it will always be 64-bit on 64-bit systems. It will have a maximum value of 2^31-1 or 2^63-1 because of one bit being lost for twos-complement negative numbers.
• UV - an "unsigned value" which likewise can be 32-bit or 64-bit, but lets you go up to the full 2^64-1 maximum value. This does not let your numbers go more negative, only more positive.
• NV - either a 64-bit floating point (IEEE-754) or 80-bit floating point. 64-bit is much faster so most systems compile it with that. 64-bit floats can hold up to 53-bit integers. 80-bit floats can hold up to 64-bit integers.

So, on a 32-bit system configured for 32-bit IV, the 64-bit NV lets you store larger integers in a float even though NV is meant to be used for fractional values. On a 64-bit system, the IV is always 64-bit so there's no advantage to using NVs to store integers.

You can check the details of how your perl was compiled using the Config module.

perl -E 'use Config; say $Config{ivsize}; say $Config{nvsize}'

FWIW, the only 32-bit system I still have access to has a perl compiled with 64-bit IV.