$x is some attribute of some node. Associated with that attribute is a (possibly empty) list, @kabluther of callbacks to other nodes associated with this node by this attribute.

Sorry, you misunderstood me. @kabluther has nothing whatsoever to do with $x or callbacks. It's just an example of some sort of processing that might occur. The example could also be

onTime {
  our $pos : Attr('position');
  our $active;
  if ($active) {
    $pos->[0]++;
  }
}
[download]

or

onTime {
  our $x : Attr('something');
  our @fixups;
  foreach (@fixups) {
    $x += $_;
  }
  @fixups = ();
}
[download]

The "callbacks" I was referring to are behind the scenes. Here's an example of the previous code sample, without using the new "simplified" API:

onTime {
  my $attr = $NODE->getAttr("something");
  my $val = $attr->getValue();
  foreach (@fixups) {
    $val += $_;
  }
  @fixups = ();
  $attr->setValue($val);
  $attr->doCallbacks();
}
[download]

That's how it would be written with the current API, unless they wanted to suppress the possibly unneeded callbacks, in which case they would spell it:

onTime {
  my $attr = $NODE->getAttr("something");
  my $val = $attr->getValue();
  my $was_updated = 0;
  foreach (@fixups) {
    $val += $_;
    $was_updated = 1;
  }
  @fixups = ();
  if ($was_updated) {
    $attr->setValue($val);
    $attr->doCallbacks();
  }
}
[download]