One solution is to pass-through the made value instead of stringifying. There may be a better way of doing this (update: the post by moritz below is that better way), but this at least works:

   token value {
      <val=object> | <val=string> | <val=number> 
    | true    | false     | null 
   }
   # ...
   given ~$/ {
       when "true"  {make Bool::True;}
       when "false" {make Bool::False;}
       when "null"  {make Any;}
       default { make $<val>.made; }
   }
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Though it would probably be better to pass-through if $<val> is defined at all rather than rely on the stringified fall-through:

    return make $<val>.made if $<val>;
    given ~$/ {
       when "true"  {make Bool::True;}
       when "false" {make Bool::False;}
       when "null"  {make Any;}
       default { die; }
    }
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