I've spent a good part of the past week answering Pugs legal questions. Still haven't cleared up all the confusion there, but we're close.
So why is this licensing stuff important? Well, part of Larry's original intent with the Artistic License was to allow proprietary and free versions of Perl to peacefully coexist. It would be great if everyone would peacefully coexist without encouragement, but since this world isn't always nice, we need other tools. The Artistic License helps by limiting the ways people can release a proprietary version, and also by saying what it takes to be considered a "free" version.
To do this it relies on copyright law, the same as the GPL and most other software licenses. The word "copyright" can be loosely translated into plain English as "an artist's right to copy or change their work, or to give it to someone else". In the commercial world, copyright law, is sometimes used to take away freedoms (which is unfortunate). But in the open source/free software world we use copyright law to promote greater freedom. (Hence the term "copyleft" which is a pun on the fact that the word "right" is sometimes used to refer to conservative political views, and "left" to more liberal ones.)
So, if you're wondering why someone would object to having their code released as public domain, or why it's worth spending a week or so to get the terms right, it's this: we want to protect and promote the freedoms of our users.