Which one of the below three examples would you prefer?

It depends :-)

sub sg {
  if (COND) {
    return 1;
  } else {
    return 2;
  }
}
[download]

I'd do this if both paths through the code were equally likely.

sub sg {
  if (COND) {
    return 1;
  } else {
    return 2;
  }
  die "should never reach this";
}
[download]

I'd never do this. I'd trust my test suite to catch any stupid code changes I might make rather than littering my code with die statements.

sub sg {
  if (COND) {
    return 1;
  }
  return 2;
}
[download]

I'd do this if 2 would be the "normal" result - although I'd probably write it as

sub sg {
  return 1 if COND;
  return 2;
}
[download]

As another option - if this was a method in some OO code I'd be looking to see whether that if statement was a sign that I was missing some polymorphic behaviour somewhere that I could factor out into separate methods.