The checksum spec is a little vague. It could be:

my $packed = pack 'NN', time(), 123456789;;

my $c = chr(0); 

$c ^= substr $packed, $_, 1 for 0 .. 7;;

$packed .= $c;;

print unpack 'C*', $packed;;
78 121 55 23 7 91 205 21 147
[download]

Or:

my $packed = pack 'NN', time(), 123456789;;

my $c = substr $packed, 0, 1; 
$c ^= substr $packed, $_, 1 for 1 .. 7;;

$packed .= $c;;

print unpack 'C*', $packed;;
78 121 66 151 7 91 205 21 102
[download]

Probably the latter.