sub fibonacci ($n ...
# Wait, wait... hold on a second! What is this $n thing you claim
# to be sending me?! Is this a thing? Is it the kind of thing that
# I approve of? What about my friends? Will my friends respect
# this thing? Let's ask them... Hold on, now, do I trust my
# friends? Maybe not. Better take some precautions...
# Time for some state management in the form of eval. That's light
# weight, right?
# Also, now is a good time to pre-format some error messages just
# in case. Okay, I think this $n thing is up to snuff, but just in
# case, I'm going to do some state management. Hold on a bit...
) {
    return 1 if $n <= 1;
    return fibonacci($n-1) + fibonacci($n-2);
}