Unfortunately, autovivification doesn't auto-share, so you have to manually create shared elements.

#! perl -slw
use strict;
use threads;
use threads::shared;
use Data::Dumper;

my %hash : shared;
my $var = 'hello';

$hash{ foo } = &share( {} );
$hash{ foo }{ bleh } = 'test';

$hash{ $var } = &share( {} );
$hash{ $var }{ foo } = 'test';

print Dumper \%hash;
__END__
P:\test>test
$VAR1 = {
          'foo' => {
                     'bleh' => 'test'
                   },
          'hello' => {
                       'foo' => 'test'
                     }
        };
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Note: The &shared( {} ); syntax is required. See threads::shared for details.