The reason bignum most likely wants to be as pure perl as possible is because this means that anyone who can run perl can run bignum, regardless of their access to a compiler etc.
The big advantage is that for instance a hosting provider that does not allow compiling of code on their machine (any smart one will not let you do that) will still let you install bignum, which might be slow but still is handy.
Said smart hosting provider will almost certainly let you upload binaries though, so you could compile your modules (and most likely you're own perl as well) and upload that to your site dirs... If you couldn't do *that* it'd be time for a new one I imagine. Not that I've ever been in that kind of shared hosting. You can get virtual hosting for as little as $7/m these days...