Unary dot and custom control

[Luke Palmer]

I came across this slight annoyance working in Perl 5 today:

    sub preserve(&) {...}

    sub foo {
        preserve {
            $_[0]->bar;
        }
    }

That didn't call "bar" on the invocant of "foo", but rather on "undef",
because preserve's block was a hiding sub.

Perl 6 is making it easier to define custom block constructs like this
one.  I worry about:

    method foo () {
        preserve {
            .bar;
        }
    }

This one's a tad more subtle.  Normally preserve's block would be 0-ary.
But in this case it's unary, since it implicitly references $_.  

Is there a way to declare preserve so that its block will always be
parsed 0-ary?  "if" is certainly able to do this (consider the above
example with s/preserve/if $baz/).

Luke