It's absolutely still relevant, but it might not be as dominant as it was when first released. By that I mean that the landscape of software development has changed significantly with much more community-driven ecosystems (no longer ~just CPAN) and much more programming is figuring out how to think about choosing the right off-the-shelf architectural components.

So, it's relevant and still excellent in guiding you how to write and manage code, but make sure you don't let it lead you to a conclusion that you should write your own code for all aspects of your system.