In my experience, Perl has inherent problems with readability on a line-by-line basis. I don't think that there's anything that can be added to change this - testing would prove a line does certain things but given that a test isn't complete specification, testing couldn't by itself change the readability problem.
Again, Perl would have more justification if Ruby wasn't visible
If someone references a book with author/title, PLEASE at least check a review on the web before commenting.
I was hoping that any answer might be insightful.
(Readability is mostly a function of familiarity. Everything has advantages/disadvantages, if I'd list the problems with Perl, readability wouldn't be there.)
Details and exact choices of libraries can be discussed, but... could you be more specific?
Or were you talking of 1990s code, before e.g. testing took over the Perl world? Nah, straw men like that on HN... not possible.