sub ccn {
    local $_ = shift;
    my @n;
    for (my $i = 0; $i < length; pos = $i++) {
        $n[length $&] = $& if /\G(.+).?(??{reverse  $1})/;
    }
    return @n ? $n[-1] : '';
}

sub ccn2 {
    local ($_, $s) = shift;
    for (my $i = 0; $i < length; pos = $i++) {
        $s = $& if /\G(.+).?(??{reverse  $1})/ and length($s) < length
+($&);
    }
    return $s;
}
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