I'd consider it a disaster if installing perl 6 uninstalled or overwrote any install of perl 5. Installing one won't automatically preclude installing the other, and I fully expect that anyone shipping a perl 6 install will ship a perl 5 install as well, at least for quite a number of years. (Much the same as when folks shipped perl 4 and perl 5 simultaneously)

I personally don't plan on running my working perl 5 programs under perl 6 for anything other than testing purposes--they work, disk space is cheapish, and I loathe breaking things without a darned good reason.