I think you want to hash it the other way, going from cluster to servers (big to small):

use strict;    # always use this
use warnings;  # .. and this
my %servers = (  # store the server cpu values by cluster
  cluster1 => {
    server1 => 10,
    server2 => 20,
  },
  cluster2 => {
    server3 => 15,
    server4 => 5,
  },
);
foreach my $clusterName ( keys %servers ){
  my $servers = $servers{$clusterName};
  next unless scalar values %$servers;   # avoid div by zero
  my $sum = 0;
  $sum += $_ for values %$servers;
  printf "%s cpu avg is %.2d%%\n", $clusterName, 100*$sum/(scalar valu
+es %$servers);
}
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