So, is this grasping the concept behind $messages{$guess <=> $answer}? which is the part I am mostly trying to understand.

hmmm.. I don't really follow your interpretation. Here is how I would explain it:

Take the result of the numerical comparison between $guess and $answer, and use that as a key to the %messages hash, then return the value associated with that key.

So, for example:
Lets say the correct answer is 10, and the user guesses 5.
Then we effectively have:

$messages{5 <=> 10}
[download]

Because 5 is less than 10, 5 <=> 10 will return -1. So that becomes:

$messages{-1}
[download]

Hope this helps,
Darren :)

PS. The spaceship operator "<=>" is generally used for sorting operations. For a good introduction to sorting, I'd thoroughly recommend this article by merlyn.