perl6 where clauses on subsets don't close over outer vars?

gistfile1.pl

use Test;
plan 2;

my $outer = "hello";
subset LikeOuter of Str where { $_ eq $outer };

my LikeOuter $a = "hello";

dies_ok { $a = "what" };

$outer = "what";
lives_ok { $a = "what" };