My math is a little rusty, but I tried your code with Statistics::Basic::Covariance:

#!/usr/bin/perl 

use strict;
use warnings;
use Statistics::Basic qw(:all);

use constant HITS  => qw(2378 4024 9696 
                      7314 7710);
use constant SALES => qw(310.94 315.88 514.15 
                      500.18 610.37);

my $cov = (HITS, SALES);

print
"The covariance is : $cov\n";
[download]

Update: Fixed---here's the corrected code:

#!/usr/bin/perl 

use strict;
use warnings;
use Statistics::Basic qw(:all);
$Statistics::Basic::DEBUG = 1;

use constant HITS => qw(2378 4024 9696 7314 7710);
use constant SALES => qw(310.94 315.88 514.15 500.18 610.37);

my $cov = Statistics::Basic::Covariance->new;
$cov = covariance( [HITS], [SALES] );

print
"The covariance is : $cov\n";
[download]