use strict;
use warnings;

my %disk_space = (
    host_A => {
        '/'     => 0.22,
        '/var'  => 0.51,
        '/opt'  => 0.79,
        '/home' => 0.17,
    },
    host_B => {
        '/'     => 0.31,
        '/var'  => 0.82,
        '/opt'  => 0.99,
        '/home' => 0.22,
    },
);

printf( "%-10s:%-10s -> %.2f\n", @{$_} ) for
sort { $a->[2] <=> $b->[2] }
map{
    my $host = $_;
    map{
        [ $host, $_, $disk_space{$host}{$_} ]
    } keys %{ $disk_space{$host} };
} keys %disk_space;