Subclassing standard types

package Radius;
require Tie::Scalar;
@ISA = (Tie::StdScalar);

use overload '*' => \&multiply;
#
# I'm not sure how this goes...

sub multiply {
    my $self = shift;
    my $other = shift;
    return $self->FETCH ** $other;
}

class Radius(int):
    def __mul__(self, other):
        return self ** other

radius = Radius(6378)
# radius acts exactly like an int, 
# except when multiplied.

radius / 2.0   # 3189.0
radius * 0.5   # 79.862...

print "Earth's circumference:", int(pi * radius * 2), "km"