The 00 in the term $hash{00} is initially evaluated as a number, rather than a string, because it's not in quotes. Perl then converts the number back into a string to store into the hash. As a result, the assignment

    $hash{00} = "data";
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actually stores into $hash{"0"}, not $hash{"00"}.

Later, when you try to access $hash{0 . "0"}, you do access $hash{"00"}, which of course is not the same as $hash{"0"}.