You bring up an interesting point, which is the question of relative traffic vs absolute traffic.

We can easily answer the question of absolute traffic.

However, let's say traffic is down. Part of that, as you say, may be because of a lapse in popularity of Perl.

However, what part of this is due to competition for what Perl related traffic there still is, like with StackOverflow, etc

Again, this is anecdotal, but I feel like before, when I googled perl problems, the majority of the results were on PM. These days, I believe a slim majority of the results now come from Stackoverflow.