I seem to recall that range semantics are implemented in terms of ++, so this:

for ($a .. $b) { ... }
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Should be roughly equivalent to

for ($_ = $a; $_ <= $b; $_++) { ... }
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You see that with ($a, $b) = (5, 1) the body of the loop won't be executed. (For strings the comparison is le, not <=, I think).

(update: added ++ where appropriate, Fletch++)