I have to agree with our AnoniMonk here, although a little more politely. It really depends how you are parsing it. Since this would indeed be a non-problem in parse-recdescent, I can assume you are not using it.
Your problem may be that you are not following the informal rules closely enough. You need some kind of 'scope', so that when you hit a THEN or an ELSE statement, your program goes 'new scope, start evaluating expressions again'. That way your program is compartmentalised and the routines don't have to know if they are in a nested structure or not.
e.g. if you hit an ELSE IF, your program should think to itself "got and ELSE, mark a label here, start evaluating normally, here's an if, call the if routine'. There's no need for it to know that it is inside an if routine already.
If you stick with the rules then issues like this become trivial (just remember to ascend from your recursive routines when you get to the end of the line).
And like our AnoniMonk says, Parse::RecDescent is fantastic for this kind of problem, but I find it makes register allocation much harder than it should be (in fact it beat me totally). With IMCC it should all be easy, though.
I didn't believe in evil until I dated it.