IANAL, but from my reading of the licenses, here is what I would expect to see the consequences of ActiveState's decision to be.

Any of their material which is included in their build and not contributed back to the core Perl distribution should be assumed to be under the Artistic License only. Any of the included material which is unchanged from the standard distribution (ie that does not have an ActiveState copyright claim on it) may be distributed by ActiveState under the terms of the Artistic License, but as the GPL is between you and the copyright holders only (the person or company who distributed to you is irrelevant), you get your choice of licenses again.

Of course this is only my reading, and not legal advice. If you want legal advice, talk to a lawyer. (BTW the reason you see this disclaimer is that you are liable if you give, even in good faith, bad legal advice. Part of what you are paying a lawyer for is to go out on a limb for you...)