The purpose of this is to provide a nice interface to programmers so that they don't have to track whether the watchers need to be notified or not. Here's some actual code in the current version of my system:

onTime {
  our $active;
  our $pos : Attr('position');
  if ($active) {
    $pos += [ 1, 1 ];
  }
}
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(I know there's a missing 'sub' before 'onTime', but that's intentional to match our actual setup. Just pretend there is a 'sub' there.)

I would like the developers to be able to write exactly the above code, but only have the 'position' watchers notified if $pos was written to. I can do whatever setup I want in either the attribute handler or code that is inserted before the entire body, and whatever cleanup or change notification is needed at the end of the body or after the subroutine returns.

In the current system, it just unconditionally assumes that any variable that could change, did. That means there is a cost for using the Attr(...) syntax, and I want it to at least feel "free" to the developers.

Sorry if the question was unclear. It's hard to figure out how much to explain so that people can understand the question (and maybe detect if I'm asking the wrong question), and how much to leave out so people don't have to wade through irrelevant detail.