Interesting piece by Tim O'Reilly about how they are monitoring computer book sales trends. It includes the following paragraph which discusses Perl.
Even though both are declining, the comparison of Perl and Python sales figures is especially interesting. Two years ago, the Python book market was approximately one sixth the size of the Perl book market. As of this week (I just checked), it was 2/3 the size of the Perl book market. While it's easy to point to the long delays in getting Perl 6 out the door as a reason, we believe that PHP (which is now twice the size of the Perl book market) has also taken over from Perl a lot of database-driven web sites. Fortunately, we're the leading publisher in both markets, with 72% share in Python, and 61% share in Perl. (As a side note, our Perl share is down from the high 80s a couple of years ago largely due to the increased sales of beginner's books like Perl for Dummies. Mike Hendrickson, my associate publisher for our Professional Books unit, who used to be the editor-in-chief at Addison-Wesley, reported the same thing for C and C++ in the 90s. After many years, AW's dominance was suddenly challenged by low-end books once the language became really mainstream. While this may not be good news for our share, it's a good sign that Perl is no longer restricted to the "alpha geeks.")