That is a pretty nice solution, except of course it consumes ...

... an entire mirror data structure, completely unnecessarily.

Why not store the values in your existing hash using references to shared scalars, and lock the individual scalars directly rather than via a proxy?

sub safe_set {
   my $k = shift;
   my $v :shared = shift;

   lock $$h{ $k };
   $h{$k} = \$v;
}
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The reasons why you can't lock individual hash elements are quite involved, but they boil down to the facts that:

• the values in a hash are not indexed directly by the keys themselves. And the keys aren't stored internally in scalars.
• the values in shared hashes aren't themselves shared scalars by default.

A full explanation would probably require the original author to explain, but it probably comes down to the path of least resistance.