In C culture, a bare return means "This is a procedure, not a function, and I'm not expecting to return anything", and so I think the absence of a meaningful value is properly conveyed by Nil in Perl 6. If the intent of a return is to allow interpolation of Slip or (), then that choice should be made explicit. Nil is a better default because it doesn't accidentally disappear in an unintended list context. In this sense, Nil can be considered the most basic failure value, designed to trip things up downstream if abused.
Roast changes: https://github.com/perl6/roast/commit/cecbb2a9a7
