How can a Perl 6 rule pass arguments to another rule?

In How can I pass arguments to a Perl 6 grammar? I passed an argument to a rule as part of a submerse. I was wondering how I'd do that completely inside a grammar. Suppose I had something like:

grammar TryIt {
    rule TOP            { \d+ <stuff> }
    rule stuff ($count) { <[ \S A..Z ]>{$count} }
    }

my $match = TryIt.parse: "123 ABCD";
say $match;

How do I pass an argument from TOP to stuff? I didn't find any examples and various guesses about parens near stuff didn't work.