As for the use of a hash of arrays, reading the post I would assume a badly chosen data sample rather than a misunderstanding.

Given the OPs description of the code: "My code below, tries to group the 'error_rate' (second column of data) based on its corresponding tag (first column of data).", in conjunction with that the second column appears to be a byte-wise mask for the first: 

AATACGGCCACCCCCCCCCCCCCCGCCCCTCCCC    
INILILFIIIIQNQQNQNLLKFKNCDHA?DAHHH

[download]

I don't think it is just badly chosen sample data. Maybe the OP will tell us which is correct?

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.
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In reply to Re^3: Memory Efficient Alternatives to Hash of Array by BrowserUk
in thread Memory Efficient Alternatives to Hash of Array by neversaint

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