That makes sense: the range ends up being [5..4], which is a no-iteration loop. What do you think should be the output?

NB: my code does not run under 5.8.1 (on a Mac). I don't know why, and I can't get 5.8.6 to compile. Grrr. I hate it when my PC works better than my Mac.

Update: and to answer my own question, I looked at what NestedLoops does. Sensibly enough, it does no iterations when there's a no-iteration loop. You can get this behavior by making the last line of nest be return((@out and @in) ? (@out, @in) : ());