But you still don't understand the compile/execute nature of Perl script execution and how that affects 'use' statements. [....] Moving the 'use' statements around does not affect the fact that modules loaded using 'use' are loaded before the script executes.

And you seem to not understand the purpose of bignum.pm.

(At least by default) bignum.pm overloads the handling of numeric constants when compiling Perl code so you get Math::BigFloat or Math::BigInt or similar objects in place of each numeric constant.

So what matters is whether or not bignum.pm is loaded before or after whatever part of the script is compiled, not executed.

BTW, a much saner approach is to just use whichever Math::Big* or similar module directly and only resort to huge (and slow) numbers in the precise cases where you need to.