>>6
Let's see

1. The $fail variable is set by translating "harbl" with the pattern kwmvlebrogha "eopq n stqye" which results in "yes ".
2. The string "WHAT what" goes through some translation and reversion, and the end result is "echo desu".
3. $fail is assigned to "yes " + the result of executing "echo desu"; $fail == "yes desu".
4. "yes desu" is executed.

So assuming you have all the necessary utilities (tr, rev, yes) this should just print an infinite amount of "desu" lines.

(Tested under bash 2.05b.0(1), dunno about other shells)