foreach $value (@listA) {
 $countsA{$value}++;
 $seen{$value} = 1;
}

foreach $value (@listB) {
 $countsB{$value}++;
 $seen{$value} = 1;
}

$diff = 0;
foreach $value (keys %seen) {
  if( not defined $listA{$value} ) {
    $listA{$value} = 0;
  } elsif( not defined $listB{$value} ) {
    $listB{$value} = 0;
  }

  $diff += abs($countsA{$value} - $countsB{$value})
}