Note, that there's a lot of modules for statistic computations on CPAN. Particularly you can use Statistics::Basic:

use strict;
use warnings;
use 5.010;

use Statistics::Basic qw(covariance);

my @hits = (2378, 4024, 9696, 7314, 7710);
my @sales = (310.94, 315.88, 514.15, 500.18, 610.37);

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