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
}
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Study that code. It's one proper implemention (of many). It uses many important concepts, such as references and general data structures. Please feel free to ask questions about how it works.