require Tie::SubstrHash;
tie %myhash, 'Tie::SubstrHash', $key_len, $value_len, $table_size;
Upon tying a new hash to this package, the developer must specify the size
of the keys that will be used, the size of the value fields that the keys
will index, and the size of the overall table (in terms of key-value pairs,
not size in hard memory). These values will not change for the
duration of the tied hash. The newly-allocated hash table may now have data stored and retrieved.
Efforts to store more than $table_size
elements will result in a fatal error, as will efforts to store a value not
exactly $value_len characters in length, or reference through a key not exactly $key_len characters in length. While these constraints may seem excessive, the
result is a hash table using much less internal memory than an equivalent
freely-allocated hash table.