This entry was adapted from some comments I wrote for a different, mostly orthogonal, journal post. I hope it's not too flamebait. If you have something to say, please try to be civil and good-natured. Here goes nothing:
Most modules on CPAN carry a licensing blurb saying something like "you can distribute this software under the same terms as Perl itself". This obviously begs the question what does it mean. Here is what it means for different Perl versions:
Early versions of perl, before it adopted the GPL were under a very restrictive loosely-defined licence.
After a while perl adopted the GPL exclusively.
Eventually, seeing that some people and companies had problems with the GPL, Larry Wall phrased the so-called "Artistic License", which was supposed to be more permissive than the GPL, but purposely phrased vaguley, and as such was found to be non-free and ergo GPL-incompatible.
In any case, the perl implementation was dual-licensed under the "Artistic License" (version unspecified) and the GPL version 2 or any later version. perl5 is still licensed under these terms.
For Parrot and for future work on Perl 6, the Artistic License 2.0 was created based on the original Artistic License. This license has been approved as a free software licence by the Free Software Foundation and found to be compatible with the GPL version 2 or Later. Parrot now carries this licence.
There's also the Clarified Artistic License which is an earlier effort to correct the original Artistic License and is the minimal set of changes to make it FSF-free and GPL-compatible. I suppose that now the Artistic License 2.0 would be preferable.
The problem is that perl up to and including perl-5.10.0 and bleadperl (which were licensed under the GPL version 2 or above but only the "Artistic License" not the "Artistic License version 1.0 or above". I've already noted that the Artistic License is problematic, but the GPL has its own share of potential problems (see also its FAQ), and is otherwise widely mis-understood, over-hyped (both positively and negatively) and disputed. For example, unlike the Artistic License, it does not allow (at least by linking or normal function calls) non-GPL-compatible code, including all non-open-source code, to make use of it.
Now, since both the "GPL version 2 or above" and the original "Artistic License" are problematic, and the term "under the same terms as Perl" may probably mean just that if it's a perl5 module (although who knows what it would means if someone would ever write a compatible Perl 5 implementation under a different software licence), then licensing a module as such is probably no longer such a good idea. There are several alternatives:
So my suggestion is that you use one of these licensing phrasings for your future work, and to re-license your old CPAN or Perl work (copyright ownership permitting) under such phrasings. It remains to be seen what will happen with perl5 itself which has 905 authors as of perl-5.10.0 many bringing in their own ownerships of the code. The Linux kernel now faces a similar problem if it would wish to adopt a different licence than the GPL version 2 (and no later version).
I suppose you can still make most use of perl5-like licensed code, in your own open-source, proprietary or in-house code, without getting sued, so I wouldn't worry too much. But it would still be a good idea to convert newer code (and code that can be easily converted) to licensing terms that are less ambigious, more usable, and that would play better with future versions of Perl.
For the record, most of my Perl and non-Perl open-source code is either Public Domain or MIT X11-licensed (which are both extremely permissive and allow sub-licensing), or if it is derived from a different code, then under the same licensing terms, but while disclaiming my explicit or implicit ownership (to allow the originator to relicense future versions of the code). The latter policy applies to my (relatively limited) contributions to perl5 where I am listed in the AUTHORS file.