Thanks. That's the explanation that clinches it for me.

I guess the distinction between using a built-in that returns a list, in a chain:

print sort split'', $string;
[download]

which works, and using a user sub which produces a list, in the same place:

print sort uniq @array;
[download]

which doesn't, is just another anomoly caused by the DWIMery of builtins.