glancing over the p5p list over on http://www.perl.com, I see that these attributes are apparently being used in a development version of perl, like so:

our $sv : shared = "foo";
[download]

in this case, the shared attribute means that when $sv is being altered (they say written to, but that seems a bit foreign) that all reads are locked out until the write is complete. You now have a method of implementing multithreaded multiprocessor vars. I can't tell you how this locking is achieved behind the scenes, but there's one possible use.

reference p5p digest for 06MAY2001.