I'm probably missing something, but why not take a copy of $x before the loop and check if it has changed afterwards?

sub onTick {
    our $x;
    my $copyX = $x;
    # Initialize and associate $x with some external value
    foreach (@kabluther) {
      $x += $_->getSkookiness();
    }
    if( $x != $copyX ) {
        notifyWatchers( $x );
    }
    return $x;
}
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That wouldn't detect serial changes that result in no change--$x += 10; $x += -4; $x += -6;--but whether that is a problem depends upon what the watchers are watching?