This merges the list of hashes by storing them temporarilly in the %merge hash and indexing them by 'domain'. You can omit the $to_merge temporary if you're willing to get a bit more ugly syntax.

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 %$domain_ref } = value
+s %$domain_ref;
    }

    return values %merge;
}
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