What is bad about it (IMHO):

• file size is unknown
• processing does not start untill you are done reading
• using at the very least three of four times the size of the file in memory
• most people will not realizse that you are reading it in memory first. If you really want to read it all at once then I would suggest reading it in an array first.
• the processing will be slower then reading it with a while and immedaitly creating a hash-element for each record. Now you read it in a temporary list, then you loop over that list, while looping over it you create a new list, and then you finally assign that list to a hash. Not that you will notice the speed/memory difference but that doesn't mean it's not there.

And with this technique you can't check (easily) for duplicate elements - but that wasn't asked

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