The issue is that the module got an NV, not a UV, IV, or numerical string. Sure, it can convert it, but it will probably not be what the caller intended. The source for almost all functions call _validate_int() on inputs, which is where the error message occurs.
Edit: The value it dies on for me has exceeded 64 bits, so there is no way the module would get the intended input. Better to die than give a wrong answer.
In theory to work with larger inputs, we should be doing the input calcs as BigInt (or Math::GMP or Math::GMPz, etc.) to make sure they're correct and scale past 64-bit, then do the gap/log($num) making sure the inputs are BigFloat.
The reason we wouldn't want to let $num be an NV for the log calc is that these scripts should scale to $num values of 20k+ digits, which won't fit in a double or long double.