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

Update: Changed to block code instead of inline.

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

And, incidentally, you don't include the mean method you invoke on line 18 - when you post, please include all methods in your posted code.

#!/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";
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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";
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What the other monks answered is correct, and here's a little addition:

The names of the variables $arraylref and $array2ref are hints that both of them are supposed to be references to arrays. \@sales is an array reference: the @ stands for "array" and \ stands for "reference". @hits, on the other hand, is just an array, not a reference.

Of course, you can't always count on variable names to hint at what they are supposed to be, but in this case it is correct.

Another hint that both of these variables really need to be references is the usage of the arrow operator -> in the line

$arraylref->[$i] * $array2ref->[$i];

The arrow operator is only used with references.

To be in the safe side, Next time when you write something like that consider adding some extra checking in your sub for example

sub covariance {
    my ($arraylref, $array2ref) = @_;
    if ( ref($arraylref) ne 'ARRAY' or ref($array2ref) ne 'ARRAY') {
     return "expecting array reference to be passed";
   }
}
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