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for my $str (qw[YES YE Y yes ye y YS ES S ys es s YEESS YYYEEs]) {
    say "$str: ", "yes" =~ /^\Q$str/i ? "ok" : "not ok";
...
s: not ok
YEESS: not ok
YYYEEs: not ok

say "ok" unless index "yes", lc $str;