Tie::Memoize (Perl in a Nutshell, 2nd Edition)

8.219. Tie::Memoize

Adds data to the hash when needed. That is, Tie::Memoize allows a tied hash to autoload all its values on first access and to use the cached value on subsequent accesses. Read accesses will result in one of the Tie::Memoize functions being called. Write accesses will act as if you're operating on a normal hash. For example:

#!/usr/local/bin/perl -w
require Tie::Memoize;
tie %hash, 'Tie::Memoize', \&fetch, $DATA, \&exists,
    {%ini_value}, {%ini_existence};
sub fetch { ... }
sub exists { ... }