Your pattern also matches 'YEESS', 'yeeeeeeeeeeeeeeeeeees', and 'YeEeEeEeSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSS'.

Probably not what you want.

There's a much simpler solution. Reverse the subject and the pattern:

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";
}
__END__
YES: ok
YE: ok
Y: ok
yes: ok
ye: ok
y: ok
YS: not ok
ES: not ok
S: not ok
ys: not ok
es: not ok
s: not ok
YEESS: not ok
YYYEEs: not ok
[download]

Or, instead of the pattern match:

say "ok" unless index "yes", lc $str;
[download]