Yes, you can call them open core now. But still many companies are using the community edition and are served well by it.
I worked closely with the technical founder and he started the pre-decessor of OTRS in the 90s while working at SUSE, and yeah of course it was in Perl!
I must say that working on the quite significant OTRS code base, with proper code conventions, 'modern' perl5 is not so bad. But when using third party libraries sometimes you'd see arcane language usage and there seems lots of magic involved...
Also, the technical founder started https://zammad.org -- the same idea but started in 2012 or so. So now it's Ruby!
Modern perl5 is ok. But the ecosystem is slowly deteriorating.