|Do you know where your variables are?|
> No, I'm thinking that it is pointless to compare references since any two copies will test as unequal.
your thinking of nested structures I'm thinking of objects. If you have instances representing something like "Employees" you don't wanna identify twins.
> so do the same test as == against the reference itself (the address).
Actually it's eq, think about the way scalars are compared.
> The user wants to find the intersection of two lists, so he would be told to pass @set1 and @set2, and optionally a &func, which defaults to built-in stringification.
I was already meditating about this, I also like the Python approach (where sets are a built-in datatype) to make the hash function operate on the basis of an "equality" method of "hashable" objects. (but I don't know how this is efficiently implemented) IIRC it's possible in Perl to overload the way objects are stringified.
>If I want two ad-hoc uses of [qw/1 2 3/] to be considered the same, stringifying the reference won't do it.
IMHO sets of "deeply compared" nested structure are better done with nested hashes. (kind of a tree search for each level of nesting)
> And we suppose that this (the key function) is expensive, so only call it once per value in each input list.
BTW: interesting read
update: fixed unescaped brackets