my $tlsamples = @series_po; # = @series_fr 

my $ok_samples = 0;
my $sum_po = 0;
my $sum_fr = 0;
for (my $i; $i < $tlsamples; $i++){

    # set up any conditions to skip outliers here
    if($series_po[$i] == 0){
        next;
    }

    # count & sum only good samples
    $ok_samples++;
    $sum_po += $series_po[$i];
    $sum_fr += $series_fr[$i];	
}
return 0 if ($ok_samples == 0);

$sum_fr = 1 if ($sum_fr == 0); 
my $avg_po = $sum_po / $ok_samples; # =150 / 5 = 30 
my $avg_fr = $sum_fr / $ok_samples; # = 10 / 5 = 2

$avg_fr = 1 (if $avg_fr == 0); # avoid div/0
my $pofr = $avg_po / $avg_fr; # = 15