One way:

use Math::BigInt;

$x = Math::BigInt->new(5000000000);
print($x->as_hex(), $/);

die("This doesn't work on negative numbers.\n")
  if $x->is_neg();

my $packed = '';
while (!$x->is_zero()) {
   $y = $x->copy()->bmod(256);
   $packed = pack('C', $y) . $packed;
   $x->brsft(8);
}

print('0x', unpack('H*', $packed), $/);  # Verify work.
[download]

Since you're packing, you're probably expecting a fixed size result. If so, this can be modified to handle negative numbers and to return a fixed size result.