Warning: I am an expert in neither perl internals or perl threads - so this explanation may be wrong :-)

I think that "stash" in this context is referring to the package symbol table (called a stash in the perl internals). Blessed references contain a pointer to the stash of the package they are blessed into.

From the threads doc I am assuming that this is copied when an object is returned - when it would be more appropriate to use the the package's existing symbol table (if one exists).