I'd use left and right shift. I don't know how this would benchmark (and I don't have the time or energy to try ;) against your version, but this looks cleaner, at least to me.
sub pack64bit {
my $i = new Math::BigInt shift();
my $j = new Math::BigInt $i>brsft(32);
my $k = $i  $j>blsft(32);
return pack('NN', $j, $k);
}
Mine (as well as yours) will break if something unexpected is passed.
Update:Anonymous Monk, otherwise known as frankied noticed that this breaks for negative numbers. Something bothered me anyway about using Math::BigInt to manipulate bits, so I did a rewrite using Bit::Vector:
sub pack_bv{
use Bit::Vector;
my $vec = Bit::Vector>new_Dec(64, shift);
return pack 'NN', $vec>Chunk_Read(32, 32), $vec>Chunk_Read(32, 0);
}
This one looks even cleaner  and better yet, it works!
