foreach (@kabluther) {
  #...
 }
 if (@kabluther) {
  # the "..." above was executed
  # i.e., your $x was modified
  $_->change_in_x() for @listeners; # sounds like this is something like what you want to do.
 }

##</code><code>##

 foreach (@kabluther) {
   $x += $_->getSkookiness() if $_; # ignore undef arguments
   # or if ref $_; # only count ref's
   # or if ref $_ and $_->can('getSkookiness'); # only count objects that have this method
 }

##</code><code>##

  sub onTick {
    our $x;
    my $changed;
    # Initialize and associate $x with some external value
    foreach (@kabluther) {
      do { # these two lines must be a block - do together, or not at all.
        $x += $_->getSkookiness();
        $changed++;
      } # if clause can go here I think, if needed
    }
    if ($changed) {
      $_->ticked() for @listeners;
    }
    return $x;
  }