"Can't happen"

• But there are other kinds of "can't happen":

        if ($foo > 0) { ... }
        elsif ($foo <= 0) { ... }

• Where's the else clause?

• "Oh, that can't happen"

• And this time the author is right

  • Unless there is a bug in the implementation or in the universe

• Both are outside the scope of the project

• Otherwise you are going down the path to madness:

        # Accumulate total payroll expenses

        # But first, make sure Perl's addition operator works correctly
        for (1..100) { $total += $_ }
        die "Addition failure" unless $total == 5050;