See bug report, this Astro::Units won't even install with modern Math::BigInt. I don't know what's the purpose of line 12, and why setting div_scale to 0 with older Math::BigInt results in 7 digits you observe in your third case, but that's direct consequence.

I'd suggest installing latest Math::BigInt, fixing line#12 (change value to 1? don't know) and see if deep recursion problem goes away.