I'm active on both PM and SO, plus several other StackExchange sites, and your hypothesis appears to be correct. I very frequently get SO/SE links near the top of my search results for technical questions, but I can't recall the last time Google (or DuckDuckGo) showed me a link to PM.

OTOH, though, the technical questions I do general searches on generally aren't Perl-specific. When I have Perl questions, I tend to go straight to searching "<relevant bits of syntax> perldoc", which will bypass both SO and PM. So I guess my experience may not be representative of what a rookie Perl user would see.