thanks for the clue. I'll try to look up the source of things.

The code in question is part of an unit test.
the module itself should know if the environment support 64 bit, and return (in the relevant function) either Math::BigInt object or simple scalar, if the system support 64 bit or not.

However, the test is written to not care. so I wrote the test using Math::BigInt, and thought that it will handle all the cases. apparently not.