my (@only_in_foo, @kv_different, @kv_equal);
foreach (keys %foo) {
   if (exists $bar{$_}) {
      if ($bar{$_} eq $foo{$_}) {
         push @kv_equal, $_ 
      }
      else {
         push @kv_different, $_ 
      }
   }
   else {
      push @only_in_foo, $_ 
   }
}
my @only_in_bar = grep { ! exists $foo{$_} } keys %bar;