Tie::HashDefaults - Let a hash have default values

       use Tie::HashDefaults;

       tie %h, 'Tie::HashDefaults', \%defaults1, \%defaults0;

     This creates a data structure which is essentially	an array
     of	hashes;	this list contains all the hashes (passed by ref)
     in	the argument list; but it also contains	a new, internally
     created, anonymous	hash.  This hash is used to store any
     insertions	into the tied hash.

     Whenever a	fetch (or an exists) is	done on	the tied hash,
     the requested key is searched for in each hash in the list,
     beginning with the	internal "storage" hash; if it is not
     found in that hash, the key is looked for in the first
     default hash, then	the next, and so on, until it is found in
     one of them, or there are none left to search.

     When an iteration (keys or	each) is done on the tied hash,
     the set of	keys returned is the union of keys from	all of
     the default hashes, along with the	storage	hash.

     For operations that alter a hash -- store,	delete,	clear --
     the default hashes	are never touched.  Only the storage hash
     is	cleared.  One effect of	this is	that if	the tied hash is
     cleared, e.g. via %h = ();, and immediately following that
     an	iteration is started (via keys or each), it is likely
     that some keys will be returned.  (Unless,	of course, there
     is	no data	in any of the given default hashes.)

     Manipulating the Defaults List

     The list of default hashes	can be manipulated directly.  To
     do	this, a	special	method on the tied object returns an
     array, by reference, containing the list of default hashes.
     Any changes to this array are reflected inside the
     Tie::HashDefaults object.	For example, to	add another
     defaults source that takes	precedence over	the others
     already on	the list:

       unshift @{ tied(%h)->get_defaults_list }, \%new_default_source;

     Or, to reverse the	order in which the defaults are

       $ar = tied(%h)->get_defaults_list;
       @$ar = reverse @$ar;

     (Once you have the	array-ref "handle" on the defaults list
     array, it's good for as long as the tied object stays tied.)

     NOTE: calling get_defaults_list also resets the iterator; so
     don't call	it within an each loop on a hash tied to this

AUTHOR (John Porter)

     This is free software.  This software may be modified and
     distributed under the same	terms as Perl itself.