if ($counter <= 8) {
        $sum_values[$x] += $ifSpeeds{ $key };
        $counter++;
    } else { 
        $x++;
        $sum_values[$x] = $ifSpeeds{ $key };
        $counter = 1;
    }
[download]

To avoid the almost-duplication of the key statement, you could also write

    $sum_values[$x] += $ifSpeeds{ $key };
    $x++ unless ++$counter % 8;
[download]

or even simply

    $sum_values[$counter++/8] += $ifSpeeds{ $key };     # the [] impli
+es int(...)
[download]

(Perl does not generate a warning when you do something like $sum += ..., with $sum being undef initially — it treats it as 0)