Something like this?

use 5.010;
use strict;
use warnings;

use List::Util 'sum';

my %messages;

$messages{one} = {
    from => "Alice",
    to   => "Bob",
    size => 1000,
};
$messages{two} = {
    from => "Bob",
    to   => "Carol",
    size => 200,
};
$messages{three} = {
    from => "Carol",
    to   => "Dave",
    size => 30,
};
$messages{four} = {
    from => "Dave",
    to   => "Eve",
    size => 4,
};

say sum map $_->{size}, values %messages;
[download]