a nested hash approach

#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;

my $mode;
my %vol;
my %host;
while(<DATA>){
next if /^\s*$/;
next if /^volume_name/;
    if (/^host_output/){
        $mode = "host";
    next;
    }
    if (/^volume_output/){
        $mode = "volume";
    next;
    }
    my ($k,$v) = split /\s+/;
    if ($mode eq 'host'){
        $vol{$k}{host} = $v;
    } elsif ($mode eq 'volume') {
        $vol{$k}{size} = $v;
    }
}
for my $k ( keys %vol ) {
    $host{ $vol{$k}{host} }{$k} = $vol{$k}{size};
}


for my $k ( keys %host ) {
    no warnings;
    $host{$k}{total_size} += $_ for values %{$host{$k}};
}

print Dumper (\%host);

__DATA__
host_output
volume_name   host_name
vol1          host1
vol2          host1
vol3          host1
vol4          host2
vol5          host2
vol2          host2

volume_output
volume_name size
vol1        10g
vol2        20g
vol3        30g
vol4        30g
vol5        20g
__END__

$VAR1 = {
          'host2' => {
                       'vol2' => '20g',
                       'vol4' => '30g',
                       'vol5' => '20g',
                       'total_size' => '70'
                     },
          'host1' => {
                       'vol1' => '10g',
                       'total_size' => '40',
                       'vol3' => '30g'
                     }
        };
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