#!/usr/bin/perl

use strict;
use warnings;
use feature qw/say/;
use Data::Dump qw/pp dd/;


my %data = (
    "key0.1" => [
        {
            "key1.1" => 3.3,
            "key1.2" => 17.8,
            "key1.3" => -2.4,
        },
        {
            "key1.4" => 5.1,
            "key1.5" => 13,
        },
        {
            "key1.6" => -69,
            "key1.7" => 127,
            "key1.8" => 2.718,
            "key1.9" => 3.3,
        },
    ],
    "key0.2" => [
        {
            "key1.10" => 3.3,
            "key1.11" => 2.5,
        },
        {
            "key1.12" => -33,
        },
    ],
);

my %flat = ();

while (my ($k0,$v0) = each %data) {
    my $k1=-1;
    for my $v1 (@$v0) {
        $k1++;
        while (my ($k2,$v2) = each %$v1) {
            $flat{$k0,$k1,$k2} = $v2;
        }
    }
}

my @sorted_keys = sort { $flat{$a} <=> $flat{$b} } keys %flat;

for my $k (@sorted_keys) {
    my $v = $flat{$k};
    say "$v \t<= \t", join ",  ", split ( $; , $k );
}
[download]

Output:

-69     <=     key0.1,  2,  key1.6
-33     <=     key0.2,  1,  key1.12
-2.4     <=     key0.1,  0,  key1.3
2.5     <=     key0.2,  0,  key1.11
2.718     <=     key0.1,  2,  key1.8
3.3     <=     key0.1,  0,  key1.1
3.3     <=     key0.1,  2,  key1.9
3.3     <=     key0.2,  0,  key1.10
5.1     <=     key0.1,  1,  key1.4
13     <=     key0.1,  1,  key1.5
17.8     <=     key0.1,  0,  key1.2
127     <=     key0.1,  2,  key1.7
[download]

HTH! =)