# Here's what you have.
sub new {
 my ($class, $callback) = @_;
 my $self  = { COUNTER => 0, HASHTYPE => TYPE0 };
 bless $self, $class;
 if ($callback) {
  $self->{CALLBACK} = \&$callback;
 } else {
  $self->{CALLBACK} = \&nocall;
 }
 return $self;
}

# Start with this instead, maybe
sub new {
 my ($class, $callback) = @_;
 $callback ||= \&nocall;
 my $self  = { 
  COUNTER  => 0, 
  HASHTYPE => TYPE0,
  CALLBACK => $callback, 
 };

 bless $self, $class;
}