Do you have any example where your $was_updated flag cannot be done away with completely and replaced with a simple test against @fixups?

onTime {
  my $attr = $NODE->getAttr("something");
  my $val = $attr->getValue();

  foreach (@fixups) {
    $val += $_;
  }
  if ( @fixups ) {
    $attr->setValue($val);
    $attr->doCallbacks();
  }
  @fixups = ();
}
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If not, I'm kinda at a loss to understand the difficulty.