The solution I used in JSON::Tiny is to have separate rules for the different literal values:

proto token value {*};
token value:sym<true>    { <sym>    };
token value:sym<false>   { <sym>    };
token value:sym<null>    { <sym> };
...
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And then in the action class, have one method per literal:

method value:sym<true>($/)   { make Bool::True  }
method value:sym<false>($/)  { make Bool::False }
method value:sym<null>($/)   { make Any }
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But the key is, as already mentioned by duelafn, to create the right object in the action directly, and not try to post-press the made value later on.