Cheating and looking at the readline code ... I wonder if $term->rl_set(ShowAllIfAmbiguous => 'On') would work.

Disclaimer: completely untested. I only have Term::ReadLine installed because I like using it with the perl debugger, not because I actually code with it. I'm posting this because no one has yet responded - not because I'm an expert. Take two aspirin and call someone else in the morning. :-)