I would guess only a very small portion of posters do a Super Search before posting. And even then, anything other than trivial questions are hard to find previous answers for. The Q&A section may have some impact though.

IIRC, there was a period where the site wasn't indexed on any of the search engines, which probably had a downward impact on traffic. That's now been rectified, which may account for the recent upturn. Other than that, I'd say things are tapering off because of other places to ask Perl questions (and get answers), and a general (but relatively slow) decrease in the popularity of Perl over the last few years.