But I suspect the real killer is when just inc/decrementing a ref count causes an entire page of memory to no longer be shared. But, despite this, I have seen some evidence of caches of Perl data staying partially shared, despite my expectation that this is way too easy to thwart. The case of each child not even looking at most of the data does make the odds improve some.

If I wanted to share lots of data between Perl child processes, I'd probably at least consider storing that data via Judy.