I'd probably use just bitwise math, but as you've asked for pack, here's one way:

$s = chr( 0xa8 ) . chr( 0x38 );;

print unpack 'b*', $s;;
0001010100011100

print unpack 'A5A4A7', unpack 'b*', $s;;
00010 1010 0011100

print map{ unpack 'C', pack 'b*', $_ } unpack 'A5A4A7', unpack 'b*', $
+s;;
8 5 28

( $day, $month, $year ) = map{ 
    unpack 'C', pack 'b*', $_ 
} unpack 'A5A4A7', unpack 'b*', $
printf "%4d-%02d-%02d\n", 1980+$year, $month, $day;;

2008-05-08
[download]

Updated: Or using bitwise math:

$s = chr( 0xa8 ) . chr( 0x38 );;

$n = unpack 's', $s;;

( $day, $month, $year ) = ( 
     $n & 0x1f, 
   ( $n & 0x1e0 ) >> 5, 
   ( $n & 0xfe00 ) >> 9 
);;

printf "%4d-%02d-%02d\n", 1980+$year, $month, $day;;
2008-05-08
[download]