build a perl that uses long doubles too

Yes, it becomes very difficult to always pick the right choice when you build things such that some numbers only fit in IVs and some only fit in NVs. Perl has a long history of all IVs also fitting into NVs and trying to fight both that history and expecting some magical perfection in Perl opcodes being able to predict whether the results should be calculated as an IV or an NV is going to lead to disappointment.