{
   my $foo = {}; # ref count 1
   $foo->{foo} = $foo; # ref count 2
} ### $foo goes out of scope here, memory should be released to perl
# here, $foo is not addressable (you can't refer to it by $foo),
# but its memory is not released until exit
# because there is still a reference to $foo 
# it refers to it self, its self-referential,  a circular reference
# for memory held by $foo to be released after $foo goes out of scope
# $foo->{foo} has to be weakened, the reference count decreased


##</code><code>##

sub once {
  my ($self, $name, $cb) = @_;

  weaken $self;
  my $wrapper;
  $wrapper = sub {
# a special kind of circular reference
# CLOSURE, $wrapper keeps $self alive after return $wrapper (REFCNT+1)
# $wrapper refers to itself, CIRCULAR REFERENCE
    $self->unsubscribe($name => $wrapper);
    $cb->(@_);
  };
  $self->on($name => $wrapper); # potential CIRCULAR REFERENCE (on probably also stores $wrapper, but it probably also uses weaken )
  weaken $wrapper;

  return $wrapper;
}