my $app = sub { ... };
builder {
    enable "Plack::Middleware::Foo";
    enable "Plack::Middleware::Bar", opt => "val";
    enable "Plack::Middleware::Baz";
    enable sub {
        my $app = shift;
        sub {
            my $env = shift;
            $app->($env);
        };
    };
    $app;
};