#!/usr/bin/perl
use strict;
use warnings;

## setup
my ($gap, $elements, $first, $i) = (10000, 20000, 10000, 0);
my @data = map { rand } (1 .. $elements);

## compute initial sum
my $sum = 0;
$sum += $_ for @data[0 .. $gap];

## go through @data serially computing averages
while (1) { 
  printf("average of %d to %d: %d",
         $i + $first, $i + $first + $gap, $sum / $gap);

  $sum -= $data[$i];                # subtract old 1st element
  last if ++$i + $gap > $elements;  # stop when done
  $sum += $data[$i + $gap];         # add new last element
}