Why did you post this as an update instead of replying to my node? I would have told you earlier that my solution does no copying.

Right on both counts, of course! I'm very sorry for not replying to your node; I was overwhelmed by the number of replies and the subtleties that came up as I tried to test them against my (un- and under-stated) expectations, and so went the lazy way of just updating the main post rather than replying to everyone individually.

It was this line:

alias my $temp = $$xr;

skimmed quickly, that made me think that there was copying, because I'm not familiar with the semantics of Data::Alias; I'm sorry for misunderstanding, and I will update my update accordingly.