However, this one forces modified works to be in the Public Domain, which is nowhere near the terms of the original work.
Since this is derived from Artisic, it is assumed freely available means the same as it does there, which includes public domain, or posting the source publicly. The key is 'publicly available', a usenet posting qualifies, as does posting to a public archive, or a public http server, or ftp server, etc.
There is a clause for that for corporations which makes it quite clear that it is also intended for individules.Moreover, forced distribution of modified versions is considered non-free (one should be allowed to create private modifications).
These are the sort of reasons why FSF considers the non-clarified artistic non-free. Some of the terms are so ambigious tat you can only guess what the author meant, and indeed you must go by your guess, or not distribute the software at all.