Last year I finished over a decade on very large Perl project, collaborating within a team. When I joined that project it was already over a decade old a most of the code was much like what you are probably imagining. When I left the project, all the parts we had touched were much like any other high quality modern codebase, and we could almost as readily maintain the decade-old parts (that we had worked on) as the week-old parts.

Having worked in many languages over many decades, I've learned that a high quality codebase is built by high quality developers, largely independent of the languages they use. You can build an unmaintainable mess in any language as easily as you can build a high quality codebase in Perl. I would concede that you can more easily build an unmaintainable mess in Perl than in many languages but that's up to you; you can do most things more easily in Perl than in many languages ;)