I think one reason is that the Perl community tends to be more oriented towards publishing modules than ready-to use applications, and Perl modules are published more often in CPAN than in sourceforge. Other communities, such as PHP, tend to do the opposite. Also consider that for many things like Windows desktop applications, C/C++ is likely to be a more practical choice than Perl. Regarding Java, some people like to blame it on the marketing. ;-)
in reply to Perl and open source
Of course, as with all generalizations, there are exceptions.