As ysth says your question is a bit vague. Anyway, here's an approach that you can start to work with.

use Hash::Flatten qw/ flatten unflatten /;

sub pair { ... } # 1,2,3,4 => [1,2],[3,4]

my %data;
$data{1} = {'a' => 10, 'b' => 5, 'c' => 20 };
$data{2} = {'a' => 90, 'b' => 15, 'c' => 40 };

my @sorted = sort { $a->[1] <=> $b->[1] } pair(%{flatten(\%data)});
print $_->[0], "\n" for @sorted[0..2];

__END__
1.b
1.a
2.b
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ihb