ideas - package / versions

package Foo v1.0.0 {
}

package Foo v2.0.0 {
    extends Foo v1.0.0;
}

use alias Foo::Old => Foo v1.0.0;
use alias Foo::New => Foo v2.0.0;

Foo<v1.0.0>->new;
Foo<v2.0.0>->new;

if ($var isa Foo::Old) ...
if ($var isa Foo<v1.0.0>) ...

# XML inspiration
package Bar provides "http://foo.bar/..." {
}