|Perl: the Markov chain saw|
I can think of a few reasons.
First, Perl has its own community and distribution methods: we use CPAN, we publish code on PM and on Perl-specific sites. Perl developers seem less likely to want or use the SourceForge network.
Second, there are things Perl just isn't well-suited to: writing drivers, for example. A great number of SF projects are Linux drivers for things.
Third, while Tk is pretty slick, it isn't the kind of toolkit that many developers and end-users would want for a polished, comprehensive GUI product. I personally like how clean it is, but many developers want to use Qt and GTK and wx* toolkits.
Fourth, C/C++ have been around longer than Perl; they also have more developers.
Fifth, one of the beautiful things about Perl is how it makes small tasks easy to automate and enhance. A lot of how Perl is used results in programs of a few hundred lines that do one particular thing really well. I can't imagine a lot of those "mini-scripts" would get published as SF projects.