Checking for $hash{a}{b} causes $hash{a} to suddenly contain the reference to an anonymous hash that is checked for the 'b'.

Taking references to the elements of shared arrays and hashes does not autovivify the elements, and neither does slicing a shared array/hash over non-existent indices/keys autovivify the elements.

use strict;
use warnings;

use threads;
use threads::shared;

my %hash :shared;

print '$hash{a}{b} ',  ( exists $hash{a}{b} ) ? "exists" : "doesn't ex
+ist";
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