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.