Well, since the guy who wrote that is in my Perlmongers group and I got the link from following a discussion on our mailing list, I feel qualified to say he is talking about any dynamic web programming. And though irrelevant to the point at hand, at least his current job is a Perl based ecommerce site. At any rate, his point is that the session object represents a global namespace. And we know what happens when you try to stuff the variables for every scope of your program into a single namespace.
It's as simple as that.
Actually, if you browse the directory above that article, you'll find a number of other short articles on web programming that I found quite insightful, esp in how he questions and rejects a few other common practices in this subject area as well.
Makeshifts last the longest.