Waddayaknow! And I've even found a use for it:

print map ${ \( ++$i ) } %= 10, 1..30;;
1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0
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Now all we've gotta do is document it, and we've a valid reason for it working that way: measuring golf games. Perfect :)

Mind you, it might make explaining this one a tad awkward :)

$i = 0; print ++${\++$i}, 0+ ++$i, 0+ ++$i, ++${\++$i};;
6 3 4 6
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The argument list to print is evaluated from left-to-right. ++${\++$i} is merely incrementing $i twice and returning an alias to $i.


$i = 0; print ++${\++$i}, 0+ ++$i, 0+ ++$i, ++${\++$i};;
              ^              ^        ^     ^
             (1)            (2)      (3)   (4)
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(1) & (4) return aliases to $i. Since $i has been incremented six times, we get 6 for both of them.

(2) & (3) return copies to $i because of the zero additions 0+. We get 3 for (2) after three increments, and 4 for (3) with one more increment.

You appear to be saying that the ability to describe what the implementation does, justifies why it does it that way.

The fact that this:

$i=0; print $n = ++$i, $m = ++$i;;
1 2
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produces a different result to this:

$i=0; print ++$i, ++$i;;
2 2
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Is just plain weird. Some, including me, would say 'broken'.

Justifying the weirdness by saying "that's what it does", kinda reminds of Apple's "You're holding it wrong." explanation of Antennagate.