sub limit_bytes {
    my ($str, $max_bytes) = @_;

    utf8::encode $str;

    if (length($str) > $max_bytes) {
        substr($str, $max_bytes+1) = '';
        $str =~ s/(?:[\xC0-\xFF]?[\x80-\xBF]+|.)\z//;
    }

    utf8::decode $str;

    return $str;
}
[download]

Using use bytes; to encode is a bad idea, but if that's what you want, don't forget to also use utf8::upgrade.

sub limit_bytes {
    my ($str, $max_bytes) = @_;

    utf8::upgrade $str;
    use bytes;

    if (length($str) > $max_bytes) {
        substr($str, $max_bytes+1) = '';
        $str =~ s/(?:[\xC0-\xFF]?[\x80-\xBF]+|.)\z//;
    }

    return $str;
}
[download]