It depends on how you write things. For example, rather than writing a function iteratively (i.e., using the loop structure), write it recursively. Catching loops is then a matter of making sure you're not trying to calculate something you've previously tried to calculate - unless there's some random stuff in your function (making it a not-function :) if you've visited the same place twice, you're in a loop.

The only thing to catch then is infinite diversions - such as, "Sum of all numbers to N" could be written:

sum nums_to_n {
   my $n = shift;
   if ($n==0) return 0;
   else return $n + nums_to_n ($n-1);
}
[download]

This obviously loops indefinitely for $n < 0, which is not good. Usually, the answer is to make sure that your base case always occurs - that is, you can guarantee that there are certain values you can always calculate, and that you only accept values which rely, ultimately, on those values you can calculate.

updated: changed a < into &lt; .. duh..