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

And the exception to this is when using threads::shared:

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.

Thus, the following results in a runtime error:

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";
[download]