Look more closely at "perldoc threads::shared". You have to declare variables as shared in order for different threads to see a given variable as referring to the shared data:

my %a : shared;
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or, alternatively:

my %a;
share( %a );
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When you stringify and print the values of different references to the same underlying hash, they will still show up with different memory address values (and I can't say as I understand why that is).

In any case, your test script does behave differently with the "share" statement included -- possibly it does what you intend.