use strict;
use threads;
use threads::shared;

my %thash : shared; ## Share the base hash
my $temp = {};      ## Get a reference to an anon. hash in a temp var
share( $temp );     ## share() the temp var
$thash{A} = $temp;  ## assign it into the structure
$thash{A}{1} = 'b'; ## At this point you have what you asked for.