These Amazon.com and slashdot.org has been mentioned again and again, probably since mid-1990s too, around the time of which PHP did not even exist.

The current Perl incarnations of Amazon, Ticketmaster, CitySearch, and LiveJournal were all written at a time when PHP existed and was very popular. Vox.com is a new site in Perl. Your guesses about Amazon are not correct -- they use a lot of C++ and some Java there for backend things, but the the front-end of the site is all Perl.

However, you say you are looking for open source applications, which these are not. There are some, Movable Type, Scoop (powering dailykos.com and many others), and Slashcode being big ones. For the most part though, this reflects a difference in culture between Perl and PHP. If you're a PHP hacker and you have some time and want to share something with world, you're likely to write an image gallery or blog. If you're a Perl hacker, you're likely to write some CPAN modules. If you're looking for where the bulk of perl activity is, that's where.