package My::SuperNumber;
use overload (
  # overload the subtraction operator
  '-' => sub {
    my($self, $other, $swap) = @_;
    my $result = $self->my_super_subtract($other);
    return $swap ? -$result : $result;
  },
);

##</code><code>##

use overload (
  # this is the cryptic name of the "numification" operator
  '0+' => sub {
    my($self, $other, $swap) = @_;
    return $self->as_number;
  },
);

##</code><code>##

use overload (
  # this is the cryptic name of the "stringification" operator
  '""' => op_stringify,
);
# and then in the XS code:
char *
op_stringify(left, right, swap)
    SV * left
    SV * right
    bool swap

  CODE:
    RETVAL = my_stringify(left);
  OUTPUT:
    RETVAL

##</code><code>##

sub as_string {
  my($obj) = @_;
  return "$obj" unless ref $obj;
  die "Give me a string, or something that pretends to be one"
      unless overload::Method($obj, '""');
  return "$obj";
}

##</code><code>##

sub as_string {
  my($obj) = @_;
  return "$obj" unless ref $obj;
  my $method = overload::Method($obj, '""')
      or die "Give me a string, or something that pretends to be one";
  # WRONG
  return $method->($obj);
  # RIGHT
  return $method->($obj, undef, 0);
}