use List::MoreUtils qw(natatime);
use List::Util      qw(sum);
use Data::Dumper;

my @sums;
my $it = natatime(8, (values %ifSpeeds));
while (my @vals = $it->()) {
    push @sums, sum(@vals);
}

print Dumper(\@sums);

__END__

$VAR1 = [
          '6200000000',
          '6200000000',
          '6200000000',
          '2600000000'
        ];