Why are you needless calling get_value twice? It doesn't come for free! Moreover, why are you calling map that way? Personally, I don't like using map throwing away its result; if I need a cycle I prefer using an explicit foreach, for example.

Anyway, my 2c:

    foreach my $entry ($result->all_entries()) {
        foreach my $attr ($entry->attributes()) {
            my @a = $entry->get_value($attr);
            $r->{$entry->dn()}->{$_} = @a > 1? \@a : $a[0] ;
        }
    }
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