The thing is, the tagging system works very differently than a normal "Google-like" search. And really, that's what you're doing. The "perl" part of your search is being converted to a tag search for perl. By default, tag searches will sort by whatever tab you had selected (for me, that's newest). Then you mix in the "utf-8" search term, which is the only piece that's actually being sorted by relevance. So all you're doing is searching for questions which are relevant to "utf-8" which also happen to be tagged with perl. On Stack Overflow, that's very unspecific and probably won't get you very good results.
Google, on the other hand, doesn't understand our tags. It's not going to convert "perl" into its associated tag, so it searches by relevance on both terms. You can do that on Stack Overflow, too! A simple search for
"perl" "utf-8" yields your expected result on page one, result nine, which is much more consistent with Google. You could also push your expected result t number one in any of these queries by sorting by votes.
Just understand that Google and Stack Overflow are not the same thing. Understanding each one takes different knowledge. Therefore, you can't expect them to both yield the same results.