!/usr/bin/perl -w
use strict;
use warnings;
my @hits = (2378, 4024, 9696, 7314, 7710);
my @sales = (310.94, 315.88, 514.15, 500.18, 610.37);

my $covariance = covariance (@hits,\@sales);

print "$covariance\n";

sub covariance {
    my ($arraylref, $array2ref) = @_;
    my ($i, $result);
    for ($i = 0; $i < @$arraylref; $i++) {
        $result += $arraylref->[$i] * $array2ref->[$i];
    }
    $result /= @$arraylref;
    $result -= mean($arraylref) * mean($array2ref);
}