The structure of the GPL is that they give you specific permission to do things prohibited by copyright. Should you do things like modify and distribute the code, then you are assumed to have accepted the GPL because it is the only way you wouldn't be breaking the law. Therefore if you distribute, you are bound by what the GPL says.

However when you are dual licensed, this assumption falls to pieces. You are distributing a modified version of the software. But you could be distributing it under either the Artistic or GPL licenses. If the GPL is unfriendly to you, then why wouldn't you be choosing to distribute under the Artistic license instead? So if anyone says, "The GPL doesn't allow this!" you just say, "I never had to, nor did I, accept the GPL. I am therefore not bound by it."

Or at least that is the theory that Larry Wall came up with. (To the best of my knowledge he invented it.)

In fact, this is exactly what ActiveState has done with ActivePerl. They explicitly state that they are using the Artistic License in their redistribution of Perl source, binaries, and modules. For their own value-adds, they use their own license-- which, IIRC, is a fairly friendly license (although most of its friendliness is due to the fact that they have to comply with either the AL or the GPL). What impact ActiveState specifically removing the GPL from the license options has, I don't want to speculate.

I've never actually perused the licenses on any of the modules I've downloaded from CPAN or in the Perl core module set, as I'm under the impression that they are all covered under the same terms as Perl (using phrases recommended by the Artistic License itself in most cases). Are there any exceptions that are notable in this regard?\

Clarification: the following is from the ActivePerl 'copyright.html' file in their installed distribution: ActiveState Tool Corp., has chosen to use all Open Source content in the ActivePerl Package under the terms of the Artistic License. Which I interpret to mean that they are explicitly choosing one of the licenses out of the dual license offering, and would guess this affects work derived from ActiveState's distribution.

I've never actually perused the licenses on any of the modules I've downloaded from CPAN or in the Perl core module set, as I'm under the impression that they are all covered under the same terms as Perl...

Not necessarily. The only licensing requirements to put a module on the CPAN is that you choose a license and allow sufficient distribution rights that it can be distributed. Not all modules have licenses, but the CPAN testers utility looks for that situation and alerts the author appropriately.

Granted, many are available "under the same terms as Perl", but some are not -- some are BSD only, GPL only, and Artistic only. Some are even public domain. You have to read the licensing section of the POD (which should be there for any module on the CPAN) to make sure.

If it's not there, politely suggest to the author to make it available.

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...)