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Author image Aldo Calpini


Tie::AliasHash - Hash with aliases key (multiple keys, one value)


  use Tie::AliasHash;

  tie %hash, 'Tie::AliasHash';
  $hash{ 'foo', 'bar' } = 'baz';
  print $hash{foo}; # prints 'baz'
  print $hash{bar}; # prints 'baz' too

  $hash{bar} = 'zab'; # $hash{foo} is changed too
  print $hash{foo}; # prints 'zab'


Tie::AliasHash creates hashes that can have multiple keys for a single value. This means that some keys are just 'aliases' for other keys.

The example shown in the synopsys above creates a key 'foo' and an alias key 'bar'. The two keys share the same value, so that fetching either of them will always return the same value, and storing a value in one of them will change both.

The only difference between the two keys is that 'bar' is not reported by keys() and each():

  use Tie::AliasHash;
  tie %hash, 'Tie::AliasHash';
  tied(%hash)->add_alias( 'foo', 'bar' );
  foreach $k (keys %hash) { print "$k\n"; } # prints 'foo'

To get the 'real' keys and the aliases together, use the allkeys function:

  use Tie::AliasHash;
  tie %hash, 'Tie::AliasHash';
  tied(%hash)->add_alias( 'foo', 'bar' );
  foreach $k (tied(%hash)->allkeys) { print "$k\n"; } # prints 'foo' and 'bar'

You can create alias keys with 3 methods:

  • pre-declaring them while tieing the hash

    The 'tie' constructor accepts an optional list of key names and aliases. The synopsis is:

      tie %HASH, 'Tie::AliasHash', 
        KEY => ALIAS,
        KEY => [ALIAS, ALIAS, ALIAS, ...],
  • explicitly with the add_alias method

      tied(%hash)->add_alias( KEY, ALIAS );
      tied(%hash)->add_alias( KEY, ALIAS, ALIAS, ALIAS, ... );
  • implicitly with a multiple-key hash assignement

      $hash{ KEY, ALIAS } = VALUE;
      $hash{ KEY, ALIAS, ALIAS, ALIAS, ... } = VALUE;

    The list of keys and aliases can be either an array reference, eg.:

      $hash{ [ 'foo', 'bar', 'baz' ] } = $value;
      $hash{ \@foobarbaz } = $value;

    or an explicit list, eg.:

      $hash{ qw(foo bar baz) } = $value;
      $hash{ @foobarbaz } = $value;

    Be warned that, with the last example, Perl uses the $; variable (or subscript separator), which defaults to '\034' (ASCII 28). This can cause problems if you plan to use keys with arbitrary ASCII characters. Always use the first form when in doubt. Consult perlvar for more information.


None by default. You can optionally export the allkeys function to your main namespace, so that it can be used like the builtin keys.

  use Tie::AliasHash 'allkeys';
  tie %hash, 'Tie::AliasHash';
  foreach $k (allkeys %hash) { print "$k\n"; }

But see CAVEATS below for important information about allkeys.


add_alias( KEY, ALIAS, [ALIAS, ALIAS, ...] )

Add one or more ALIAS for KEY. If KEY itself is an alias, the aliases are added to the real key which KEY points to.

aliases( KEY )

Returns a list of all the aliases defined for KEY. If KEY itself is an alias, returns the real key pointed by KEY, as well as any other alias (thus excluding KEY itself) it has.


Returns all the (real) keys of the hash, as well as all the aliases.

is_alias( KEY )

Returns true if the specified KEY is an alias, false otherwise (either if KEY does not exists in the hash, or it is a real key).

is_key( KEY )

Returns true if the specified KEY is a real key, false otherwise (either if KEY does not exists in the hash, or it is an alias for another key).

remove( KEY )

Remove KEY from the hash: if KEY is a real key, it is removed with all its aliases. If KEY is an alias, only the alias is removed. This is different from the builtin delete, see CAVEATS below.

remove_alias( ALIAS )

Removes the specified ALIAS from its real key. ALIAS is no longer an alias and can be assigned its own value. The real key which ALIAS used to point to is left unchanged.

remove_aliases( KEY )

Removes all the aliases defined for KEY.

remove_jolly( )

Removes the 'jolly' key from the hash. Operations on non-existant keys are restored to normality.

set_jolly( KEY )

Sets the 'jolly' key to KEY. When you set a jolly key, all fetch and store operations on non-existant keys will be done on KEY instead.


This module can generate a wonderful amount of confusion if not used properly. The package should really have a big 'HANDLE WITH CARE' sticker on it. Other than paying special attention to what you're doing, you should be aware of the following subtlenesses:

  • transitivity

    Aliases are 'transitive', and always resolve to their aliased key. This means that if you write:

      use Tie::AliasHash;
      tie %hash, 'Tie::AliasHash';
      tied(%hash)->add_alias( 'foo', 'bar' );
      tied(%hash)->add_alias( 'bar', 'baz' );

    $hash{baz} is created as an alias for $hash{foo}, not for $hash{bar} (which isn't a real key). This also means that if you later change $hash{bar} to point to something else, you haven't changed $hash{baz}:

      tied(%hash)->add_alias( 'gup', 'bar' );
      # $hash{bar} is now really --> $hash{gup}
      # $hash{baz} is still      --> $hash{foo}
  • delete

    The builtin delete function resolves aliases to real keys, so it deletes everything even when called on an alias:

      use Tie::AliasHash;
      tie %hash, 'Tie::AliasHash';
      tied(%hash)->add_alias( 'foo', 'bar' );
      delete $hash{bar}; # deletes $hash{foo} too!

    To delete an alias leaving its key intact, use the remove_alias method instead:

      use Tie::AliasHash;
      tie %hash, 'Tie::AliasHash';
      tied(%hash)->add_alias( 'foo', 'bar' );
      tied(%hash)->remove_alias( 'bar' ); # $hash{foo} remains intact
  • exists

    The builtin exists function returns true on aliases too:

      use Tie::AliasHash;
      tie %hash, 'Tie::AliasHash';
      tied(%hash)->add_alias( 'foo', 'bar' );
      print exists $hash{'foo'}; # TRUE
      print exists $hash{'bar'}; # TRUE

    To distinguish between aliases and real keys, use the is_key method:

      print exists $hash{'foo'} and tied(%hash)->is_key('foo'); # TRUE
      print exists $hash{'bar'} and tied(%hash)->is_key('bar'); # FALSE
  • allkeys

    If you export allkeys into your main namespace, it can be used as the builtin keys in the following code:

      use Tie::AliasHash 'allkeys';
      tie %hash, 'Tie::AliasHash';
      foreach $key (allkeys %hash) { print "$key\n"; }

    But note that allkeys is always a function call, so this does not work as you expect:

      foreach $key (sort allkeys %hash) { print "$key\n"; }

    You have to fool sort, or it will use allkeys as its sort routine. This can be done by providing an explicit sort routine, or forcing the result of allkeys to be interpreted as an array by referencing-dereferencing it, or with a two-step operation where you first assign allkeys to an array, and then operate on it:

      foreach $key (sort { $a cmp $b } allkeys %hash) { print "$key\n"; }
      foreach $key (sort @{[ allkeys %hash ]}) { print "$key\n"; }
      @allkeys = allkeys %hash;
      foreach $key (sort @allkeys) { print "$key\n"; }
  • the 'jolly' key

    The most potentially confusing feature of this module is the 'jolly' key. When you set a value for it, all 'unknown' keys become aliases for the jolly key. This means that you can't create new keys in the hash, because if a key does not exists, the value will be 'redirected' to the jolly key.

    We make an example of how this works and for what can be useful. Suppose you have a table of records with a 'city' field. You want to count the occurrencies for Rome, Paris and London (possibly expressed in different languages), and count every other city as 'Other'.

      tie %cities, 'Tie::AliasHash';
      $cities{['Rome', 'Roma', 'Rom']} = 0;
      $cities{['Paris', 'Parigi']} = 0;
      $cities{['London', 'Londra', 'Londres']} = 0;
      $cities{'Other'} = 0;
      while($city = get_city()) {
      foreach $city (sort keys %cities) {
          print "$city:\t$cities{$city}\n";

    A possible output for the above script can be:

      London: 4
      Other:  92
      Paris:  7
      Rome:   16

    Also note that the use of the jolly key is limited to fetch and store, it does not affect other hash operations, like exists, delete, each, keys and values.


v1.02 (11 Mar 2016)

Moved to github, using Build.PL instead of Makefile.PL, added license.

v1.01 (26 Jun 2003)

Fixed a bug in the EXISTS sub, now works as documented (thanks wk)

v1.00 (07 Mar 2001)

First released version

v0.01 (20 Feb 2001)

Original version; created by h2xs 1.20 with options

  -CAXn Tie::AliasHash


Aldo Calpini <dada@perl.it>