my @c = merge_aoh_by_domain( @a, @b );

sub merge_aoh_by_domain {
    my %merge;
    for my $domain_ref ( @_ ) {
        # less ugly
        my $to_merge = $merge{$domain_ref->{domain}} ||= {};
        @$to_merge{ keys %$domain_ref } = values %$domain_ref;

        # ugly
        # @{$merge{$domain_ref->{domain}}{ keys %$domain_ref } = values %$domain_ref;
    }

    return values %merge;
}