Try it this way:

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

my %h :shared;
$h{ 1 } = &share( {} );
$h{1}{2} = 3;

{
    lock %{ $h{1} };                 ## <<<<
    async {
        print "2:Waiting for lock";
        lock %{ $h{1} };
        print "2:lock acquired";
        $h{1}{2} = 10;
        print '2:lock released';
    }->detach;
    sleep 2;
}
print "1:Waiting for lock";
lock %{ $h{1} };
print $h{1}{2};

__END__
c:\test>junk70
2:Waiting for lock
1:Waiting for lock
2:lock acquired
2:lock released
10
[download]

It shouldn't be necessary, but it does work.