This module provides a shared memory cache accessed as a
tied hash.
Shared memory is an area of memory that is available to
all processes.  It is accessed by choosing a key, the
ipc_key argument to tie.  Every process that accesses
shared memory with the same key gets access to the same
region of memory.  In some ways it resembles a file
system, but it is not hierarchical and it is resident in
memory.  This makes it harder to use than a filesystem but
much faster.  The data in shared memory persists until the
machine is rebooted or it is explicitly deleted.