The examples would fail usage constraints with the parameterless MAIN. I could not get them to fail with any internal messages on current rakudo, either with the parameterless MAIN or with a MAIN that had a :$y.
Haven't looked as to when this got fixed. Tests added for both cases (though I had to add a sub USAGE when testing the parameterless case) in https://github.com/perl6/roast/commit/bdf67b698b, so closing as resolved.
