I would tend to go with option c (the traditional if structure) for the following reasons:

1. The vast majority of programmers - even non-perl programmers can (or should be able to) understand your intent.
2. If at some future time, you need to add some additional processing, you can slip another statement in without restructuring the code:

   if ( expr) {
       a(); # why didn't they think of this before?
       b();
   }
   [download]

3. (Updated)You can easily slip an else clause in there when someone realises that is required as well.