This works as expected (IE does not die) for any value of $foo except undef or empty string.

It did not die for those either.

therefore $@ should not get set

Why do you say that? It always gets set, either to the caught exception or to undef.

It's easy enough to work around this (mis-)behavior by putting a no-op at the end of the eval block:

It's not a no-op, it's an expression that returns a true value.

I'm curious as to why it works this way.

Confused about what a function is returning? Check the docs! eval is like a sub. The last value evaluated is returned.