my (@common, @uniq1, @uniq2);
for (keys %hash1) {
  if (exists $hash2{$_}) {
    push @common, $_;
    delete $hash2{$_};  # All that will be left in hash2 is what wasn't in hash1
  }
  else { push @uniq1, $_ }
}
@uniq2 = keys %hash2;