%usage_for = (
  'cluster1' => {
    'server1' => 5,
    'server2' => 10,
  },
  'cluster2' => {
    'server3' => 20,
    'server4' => 30,
  }
);

foreach my $cluster (keys %usage_for) {
  my $usage;
  print "Cluster Servers\n";
  foreach my $server (keys %{$usage_for{$cluster}}) {

    $usage += $usage_for{$cluster}{$server};
    print "\t$server => " . $usage_for{$cluster}{$server} . "\n"; 
  }
  print "Usage for $cluster => " . $usage . "%\n";
}