|Just another Perl shrine|
Other answers have described the copy-on-write of memory in Unix and the fact that you need to use shared memory (SYSV SHM stuff or mmap'd files) and a good thing would be to use a wrapper around these things like Apache::Sharedmem.
Shared memory is tricky stuff in a similar way that threads are tricky things, since you open yourselves up to race conditions where two processes are altering the shared memory and violate each other's assumptions.
As a simple example, a process might increment a variable held in shared memory by 1 and assume that it has that value later on in the same routine, whereas another process might have incremented it in the meantime. Hard-to-find bugs which are cured by adding locks (semaphores, mutexes, whatever) to define critical sections of code which only one process at a time may execute. Ugh.
There might be a simpler solution for you though. You mention that you want changes to your data to go directly to persistent store (i.e. on disk) but you also want your data to live in memory.
I'll assume that you want the data in memory for performance reasons - i.e. you don't want to suffer a disk access per-request. But...operating systems are smart and if you have sufficient RAM on your box (say, for example, enough to build the data structure you were talking of) and you are repeatedly accessing this data then the OS should keep it all nicely in cache for you. So whilst you might be accessing hash values in a GDBM tie'd hash, the OS is doesn't bother to touch disk. When you change data, the OS has the job of getting it to disk. If your data store is a relational database similar arguments apply.
The nice thing about this is that you get it for free. You still need to be careful in that different processes may change the underlying data store at a time which might be inconvenient for the other processes - this is where atomic transactions on databases come into play...
There might be other reasons why you want the memory structure, but I thought it was worth a thought.