I wrote a new Essay about what makes software applications high-quality, as well as which parameters and methods, while desirable and quality-enhancing, are not exactly quality. It was inspired by a post on a public mailing list I set up, and Perl and perl 5 are mentioned there a few times, as are many other open-source projects.
It is available in several formats: HTML to read online (on Page), DocBook/XML source (which can in turn render other formats), PDF (please don't print it even though you are legally allowed to). The licence is CC-by-2.5. Comments are welcome.