It does not use pack/unpack, but does this return the encrypted string you need?

use strict;

$_ = "639D879F224CA2A1BA73CC4884DBD362";
my @hexdata = /(\w{2})/g;

my $str = join('', map { chr(hex $_) } @hexdata);
print "$str\n";
[download]

Maybe, seeing this, someone may be able to give you a pack/unpack solution.