Yes, that's true.

Then, if you need a solution that will give you

+{
          'b' => 'B',
          'a' => 'A',
          'c' => 3
        };
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from the above data, the best is probably a straightforward solution like

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

{
        my %thash= (map (($_,$_), values %hash1), %hash2);
        @hash3{keys %hash1}= @thash{values %hash1};
}
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is not only difficult to read, but also inefficent depending on your data.