If you need to ensure that both hash-keys and hash-values are unique, then you probably need to use two hashes.   The second hash should contain an entry for every value that’s been used in the first one:   the associated value (in the second hash) doesn’t matter, as we only care whether-or-not the key (in the second hash) exists.

  if ( (!exists($$hash1{$key} && (!exists($$hash2{$value}) ) {
    $$hash1{$key} = $value;
    $$hash2{$value} = 1;    // DON'T CARE
  }
  else ...  //  (KEY AND/OR VALUE IS NOT UNIQUE...)
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Note that $$hash{$key} is equivalent to $hash->{$key} ... take your pick.