A bisect confirms the change is due to the fixing of bug RT#96008 in 639dfab: before v5.16.0, use and require were affected by the open pragma. Since your index_req.pl is apparently encoded in UTF-8, it does require a use utf8; at the top, because that's basically what the utf8 pragma means: "this lexical block is encoded in UTF-8".

Sorry, but the best practice here is to put the use utf8; pragma at the top of every Perl source file that is encoded in UTF-8, everything else is a hack, or workaround at best.