First, postmodern simply means that someone doesn't throw out the old for the sake of the new. It means that the best of old and new concepts are weighed and the best of each is used. It's a term meaning a person or their influence on a project has gotten past the modernist idea of newer necessarily being better. Perl's full of that kind of influence.

Second, if you think nobody ever called Perl postmodern before, you've not read the transcript of one of Larry's talks called "Perl, the first postmodern programming language". That insightful talk not only tells a lot about Perl, about Larry, about how Larry thinks about Perl, and about his postmodern attitude towards programming language design. It also has one of the best descriptions of postmodernism I've read.