$free_space = {
                 "node1" => {
                               "cores" => 12,
                               "mem" => 200
                            },
                 "node2" => {
                               "cores" => 12,
                               "mem" => 500
                            },
                 ...
}

##</code><code>##

cores   memory   node
-----   ------   ----
12      400      node456
12      400      node534
12      350      node23
11      500      node12
11      200      node3
10      900      node10

##</code><code>##

my %CoreMemNodes = (); #HoA <free_cores> -> <free_mem> -> (node1,node2,..)

foreach my $node (keys %free_space) {
    push @{ $CoreMemNodes{$free_space{$node}{'cores'}}{$free_space{$node}{'mem'}} }, $node;
}

foreach my $free_cores (sort { $b <=> $a } keys %CoreMemNodes) {
    foreach my $free_mem (sort { $b <=> $a } keys %{ $CoreMemNodes{$free_cores} }) {
        foreach my $node (sort @{ $CoreMemNodes{$free_cores}{$free_mem} }) {
            print "SORTED:$free_cores $free_mem $node\n";
        }
    }
}