use 5.14.0;

my @data = map { unpack "C", $_ } ("\xA3", "\x05", "\x00", "\x00", "\x00", "\x00", "\x00");

sub checksum {
    use integer;
    my $sum16bit = 0;
    $sum16bit = ($sum16bit + $_) & 65535 for @_;
    my $sumdivmod = ($sum16bit >> 8) + ($sum16bit & 255);
    return ((($sumdivmod >> 8) + ($sumdivmod & 255)) & 255);
}

say checksum(@data);