Excerpt from: http://use.perl.org/~Allison/journal/29439

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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.)
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Allison