Perl 5 version 8.0 documentation



Hash::Util - A selection of general-utility hash subroutines


  1. use Hash::Util qw(lock_keys unlock_keys
  2. lock_value unlock_value
  3. lock_hash unlock_hash);
  4. %hash = (foo => 42, bar => 23);
  5. lock_keys(%hash);
  6. lock_keys(%hash, @keyset);
  7. unlock_keys(%hash);
  8. lock_value (%hash, 'foo');
  9. unlock_value(%hash, 'foo');
  10. lock_hash (%hash);
  11. unlock_hash(%hash);


Hash::Util contains special functions for manipulating hashes that don't really warrant a keyword.

By default Hash::Util does not export anything.

Restricted hashes

5.8.0 introduces the ability to restrict a hash to a certain set of keys. No keys outside of this set can be added. It also introduces the ability to lock an individual key so it cannot be deleted and the value cannot be changed.

This is intended to largely replace the deprecated pseudo-hashes.

  • lock_keys
  • unlock_keys
    1. lock_keys(%hash);
    2. lock_keys(%hash, @keys);

    Restricts the given %hash's set of keys to @keys. If @keys is not given it restricts it to its current keyset. No more keys can be added. delete() and exists() will still work, but it does not effect the set of allowed keys.

    1. unlock_keys(%hash;)

    Removes the restriction on the %hash's keyset.

  • lock_value
  • unlock_value
    1. lock_key (%hash, $key);
    2. unlock_key(%hash, $key);

    Locks and unlocks an individual key of a hash. The value of a locked key cannot be changed.

    %hash must have already been locked for this to have useful effect.

  • lock_hash
  • unlock_hash
    1. lock_hash(%hash);

    lock_hash() locks an entire hash, making all keys and values readonly. No value can be changed, no keys can be added or deleted.

    1. unlock_hash(%hash);

    unlock_hash() does the opposite of lock_hash(). All keys and values are made read/write. All values can be changed and keys can be added and deleted.


Michael G Schwern <> on top of code by Nick Ing-Simmons and Jeffrey Friedl.


Scalar::Util, List::Util, Hash::Util