in reply to Is there some universal Unicode+UTF8 switch?
it contains unicoded Cyrillic literals
So your script itself is in UTF-8?
it outputs unicoded Cyrillic
And you are selecting UTF-8 output... are you correctly declaring the output to be UTF-8 in the HTTP headers?
it queries for unicoded JSON Cyrillic
it receives and handles unicoded JSON Cyrillic (not UTF8-encoded sequences) like this: https://ru.wikipedia.org/w/api.php?action=query&format=json&formatversion=2&list=allusers&auactiveusers&aufrom=Б
This last item raises a big question: If the JSON Cyrillic is not UTF-8, what encoding is it in? You may need the Encode module to perform character set conversion prior to JSON decoding.