Given that your provided host_output file shows a many-to-many mapping, the choice on that front would seem to be a hash of arrays (perllol), keyed on whichever value you'll want as your primary (I'm guessing host). The second half is easier, as your volume content can be stored in a simple hash. You could calculate the host size something like this:

use strict;
use warnings;

my %volume = (host1 => ['vol1',
                        'vol2',
                        'vol3'
                        ],
              host2 => ['vol4',
                        'vol5',
                        'vol2'
                        ],
             );    

my %size = (vol1 => 10,
            vol2 => 20,
            vol3 => 30,
            vol4 => 30,
            vol5 => 20,
            );

my $host = 'host1';
my $total_size = 0;
foreach my $vol (@{$volume{$host}}) {
    $total_size += $size{$vol};
}
print "$host has $total_size at it's disposal.\n"
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Update: Forgot a doc tag in my perllol link, so linked to the wrong documentation. Oops and fixed.