+{
          'b' => 'B',
          'a' => 'A',
          'c' => 3
        };

##</code><code>##

while (my ($key, $val)= each %hash1) {
        exists $hash2{$val} and
                $hash1{$key}= $hash2{$val};
        }

##</code><code>##

{
        my %thash= (map (($_,$_), values %hash1), %hash2);
        @hash3{keys %hash1}= @thash{values %hash1};
}