Glad to hear you're having fun. :)

if we were to create a class with a publicly writable attribute

All attributes are private. No matter what. All public access is via public methods. Another comment covers this in more detail.

I can't quite get it to work now:

After some digging I've concluded that one should not use `method` as the routine declarator for the value of the FETCH and STORE arguments to the Proxy but rather `sub`:

# This is NOT a good way to validate for this use case.
# But this code works in my current Rakudo.

class Interval {
    has $.lb = die 'Lower bound is required';
    has $.ub = die 'Upper bound is required';

    method lb() is rw {
        return Proxy.new:

            # note `sub` declarator and ignored arg:
            FETCH => sub ($) { return $!lb; },

            # note `sub` declarator and ignored (first) arg
            STORE => sub ($, $lb) {
                die "Require lb <= ub" unless $lb <= $!ub;
                $!lb = $lb;
            };
    }

    method ub() is rw {
        return Proxy.new:
            FETCH => sub ($) { return $!ub; },
            STORE => sub ($, $ub) {
                die "Require lb <= ub" unless $!lb <= $ub;
                $!ub = $ub;
            };
    }
}
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Using the `method` declarator as a FETCH or STORE routine isn't useful because the self in the called routine (passed in as the first arg, corresponding to the anonymous `$` parameter in the above declarations) would not be the enclosing class.

Update Has been discussed on #perl6. jnthn landed a fix for a couple of related bugs a few hours later. Your code still won't work though; fixing it will apparently have to wait for now.