I used @seen {$n,} = (); and not $seen {$n} = undef; because I used the slice in the other case as well. I wanted to use as much identical syntax as possible, and didn't see a reason to special case adding a single key.

As for using a trailing comma, all I can say is that using -w is useful. If you use it, you'll see why the comma was used.

-- Abigail