# code above package foo unchanged

package foo::alpha;
BEGIN { @foo::alpha::ISA = qw( foo alpha ); }

package foo::beta;
BEGIN { @foo::beta::ISA = qw( foo beta ); }

package foo;
sub new{
    my $class = shift;
    my $base = shift;
    return bless {}, "$class\::$base";
}

# all other code unchanged