The reason I'm here, though, is to ask: how would you design something like this (i.e., the first paragraph), and why?

:)

I'd perform some OOAD

You can't analyze design through the peephole that is your question; its too abstract?

A method that returns a String frog or Squirrel nuts

Sometimes a user may want just the String, but sometimes a user wants the nuts

Its almost as if its a good idea to return some kind of Object thats a string by default, but also an object you can ->chew cause its nuts

Sounds like a different kind of contextual return to me

As a case study on contextual return we can learn from the functional/methodial CGI's param , the classic demonstration that context sensitive return is too much cognitive load for the new and old alike, providing millions of pitfalls, and zero benefits

->non_idempotent_string

->non_idempotent_nuts

->non_idempotent_string_nuts

sub non_dempotent_string {
   my( $string, undef ) = shift->non_idempotent_string_nuts (@_);
   return $string;
}
[download]

I think thats it, more methods .... is it?