Simpler: 

for my $t (1..2) {
    threads->create(sub {
        while (1) {
            my $s = join '', sort keys %testOne;
            print "$t\t$s\n" if $s ne 'ABCDE';
        }
    });
}

sleep;

[download]

You are both reading and modifying the hash — specifically its iterator — in multiple threads. You need a lock.

In reply to Re: Inconsistent results with keys on thread shared hashes by ikegami
in thread Inconsistent results with keys on thread shared hashes by izomiac

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