1. Do i need to use lock $hash{foo} ; or lock ( \%hash ) ; if foo is used as shared_clone ?

It depends upon what you intend to modify.

When you have nested hashes so:

%hash :shared = &share( {} );
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1. To add a new key/value pair; or delete a key/value pair; or modify the value associated with an existing key in %hash you need:

   lock %hash;
   $hash{ existingkey } = newvalue;
   # or
   $hash{ newkey } = newvalue;
   #or 
   delete $hash{ existingkey };
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2. To modify the nested anonymous hash associated with the key 'foo', you need:

   lock %{ $hash{ foo } };
   $hash{foo}{somekey} = somevalue;
   # or
   delete $hash{ foo }{ somekey };
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   Ie. lock %{ $h{1} } says lock the hash referenced by the value at $h{1}.