Yeah, the Windows cmd console is very primitive, indeed. You can still copy and paste: click on the icon at the top left corner of the console windows, and you get a menu. Chose 'modify' and you get a sub-menu in which you can select, copy and paste, very unpractical, but workable.

On your original question, I am not very clear on what your problem is, but maybe you should consider bigint or Math/BigInt rather than Math::BigFloat.