my %clusters = (
    cluster1 => [ 'server1', 'server2' ],
    cluster2 => [ 'server3', 'server4' ],
);

my %averages;
while ( my ($cluster, $machines) = each %clusters ) {
    my @values;
    foreach my $machine ( @$machines ) {
        push @values, # Whatever you do here for $machine
    }
    $averages{ $cluster } = # Whatever you do here with @values
}