Foo.pm

module Foo::EXPORT::DEFAULT { }; # work around a different rakudobug
module Foo {
    our @a;
    ::Foo::EXPORT::DEFAULT<!sub_trait_broken> = sub ($trait, $block, $arg) {
        @a.push({:block($block)});
    };
}

t.pl

use Foo;

sub lol is broken {
    # Remove the 'return' to change it into an implied return, and it works.
    return "hi";
}

say @Foo::a[0]<block>();