How do they do that? Quite easily. Perl is dual-licensed under the GPL and the Artistic License - receiver's choice. ActiveState chooses the Artistic License. If you don't agree with those licensing terms, then you are free to not contribute to Perl...

I'm not a lawyer, this is not legal advice, blah blah blah.

First off, the GPL allows anyone to sell the work in question - they just have to provide the source and allow everyone else the same rights. I could sell you the linux kernel right now for $10000 (or any other amount) and I could still be in compliance with the license (I'll even give you a 10% discount if you buy within the next 10 minutes!).

Secondly, Perl is dual licensed, under both the GPL and the artistic license, right? So what does that mean? It means that I can choose which one I accept (I'm a little sketchy here, but that's my interpretation, corrections are most welcome). So for someone looking to 'acquire' Perl, the GPL is irrelevant. Like a windows machine on a network, the artistic license is a weaker point of attack.

But like I said, don't lose any sleep over it :)