I did this for unpack:

 my ($int1,$int2)=unpack('NN',shift());
    my $sign=($int1&0x80000000);
    if($sign) {
        $int1^=-1;
        $int2^=-1;
        ++$int2;
        $int2%=2**32;
        ++$int1 unless $int2;
    }
    my $i=new Math::BigInt $int1;
    $i*=2**32;
    $i+=$int2;
    $i=-$i if $sign;
[download]

update:
shamelessly (thank you!) stealing ideas from tye's unpack, I would now write down a pack like:

my $i=new Math::BigInt shift();
my($int1,$int2)=do {
  if ($i<0) {
    $i=-1-$i;
    (~(int($i/2**32)%2**32),~int($i%2**32));
  } else {
    (int($i/2**32)%2**32,int($i%2**32));
  }
};
pack('NN',$int1,$int2);
[download]