See the current Perl documentation for Tie::SubstrHash.
Here is our local, out-dated (pre-5.6) version:
Tie::SubstrHash - Fixed-table-size, fixed-key-length hashing
tie %myhash, 'Tie::SubstrHash', $key_len, $value_len, $table_size;
The Tie::SubstrHash package provides a hash-table-like interface to an array of determinate size, with constant key size and record 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
elements will result in a fatal error, as will efforts to store a value not
$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.
Because the current implementation uses the table and key sizes for the hashing algorithm, there is no means by which to dynamically change the value of any of the initialization parameters.