if you want a good answer, you need to be careful to phrase the question well. the original question talks of 6000 records, but then the example uses 5 million records.
as everybody has been trying to get across...the buffering in perl, and in all computer languages and operating systems that support buffering, has to do with input/output, and absolutely nothing to do with memory structures.
also the example code creates a rather useless (huge) array of elements that can be computed on the fly, without ever creating an array in the first place. was this the intention of the question?