1. Where is the line, for you, between "need named arguments" and "don't need named arguments" ? 2 args ? 4 ? 27 ?

For me it's less a matter of number than it is about how the arguments will be used. If they'll be used to add to or override someone else's values, then named args are the way to go. This lets me write, for one contrived example,

  sub foo {
    my ($self, %args) = @_;
    $self->bar(
      answer => 42,
      %args,
    );
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This lets foo have a dialog with bar that involves some overridable default behavior. A more common form of this is default constructor state.

  sub new {
    my ($class, %args) = @_;
    return bless {
      loglevel => 0,
      die_randomly => 0,
      %args,
    }, $class;
  }
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Here, the constructor provides default state that clients can override. Easy to do with named arguments; difficult to impossible with positional args.