Perl is free. The full source code and documentation are free to copy, compile, print, and give away. Any programs you write in Perl are yours to do with as you please; there are no royalties to pay and no restrictions on distributing them as far as Perl is concerned.
It's not completely "public domain," though, and for very good reason. If the source were completely public domain, it would be possible for someone to make minor alterations to it, compile it, and sell it-in other words, to rip off its creator. On the other hand, without distributing the source code, it's hard to make sure that everyone who wants to can use it.
The GNU General Public License is one way to distribute free software without the danger of someone taking advantage of you. Under this type of license, source code may be distributed freely and used by anybody, but any programs derived using such code must be released under the same type of license. In other words, if you derive any of your source code from GNU-licensed source code, you have to release your source code to anyone who wants it.
This is often sufficient to protect the interests of the author, but it can lead to a plethora of derivative versions of the original package. This may deprive the original author of a say in the development of his or her own creation. It can also lead to confusion on the part of the end users as it becomes hard to establish which is the definitive version of the package, whether a particular script will work with a given version, and so on.
That's why Perl is released under the terms of the "Artistic" license. This is a variation on the GNU General Public License which says that anyone who releases a package derived from Perl must make it clear that the package is not actually Perl. All modifications must be clearly flagged, executables renamed if necessary, and the original modules distributed along with the modified versions. The effect is that the original author is clearly recognized as the "owner" of the package. The general terms of the GNU General Public License also apply.