You can also make a doubly-linked list in C more memory efficient by making your nodes have the xor of the pointer you are coming from or going to. While traversing the list you always know what one of the 2 pointers is, so this allows you to go either way down the list.

This kind of thing is fun to know about, but more than slightly irrelevant to practical programming these days.

Besides which, I agree with Programming Perl. Sure, there are lots of special cases where you can not have a temporary at all. But in Perl I notice that you don't handle the case of swapping variables of different types, or variables that are references. In C you may likewise have more fun if you try finding a trick for swapping structs. (There may be one, but if so then I don't know it off-hand.)

So in general in some languages you need to write a temporary to swap. In Perl you don't write it.