It's because the other threads don't know anything about the data space where the hash was created, so having a pointer (ie. reference) to the hash is meaningless.

Think of it this way -- you're a Perl program (or even just a separate thread), and I give you the data:

    HASH(0x804c8d4)
[download]

Can you tell what the original values of the hash are?  Certainly not -- that information has been lost.  All you now know is the address of the hash, but it's in my memory space, so it doesn't allow for the reconstruction of the data.