I think there may be some justifiable confusion between something like the "sgn()" function in BASIC, which does work just like the spaceship operator (returning -1 or 0 or 1), and something else that is more like the "copysign" function in C. There's probably a natural tendency to want to do something like this:

$delta_to_add = $magnitude * $direction;
$value += $delta_to_add;
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where direction is "1" for "up" and "-1" for "down"; but imagine a case where $magnitude is not zero, and $direction could be derived from or based on any value whatsoever, such that anything non-negative should count as "positive". You want to "copy" the sign bit from $direction onto $magnitude, but Perl doesn't really have a "copysign" function, and your "standard sgn()" function (like the spaceship operator) would do the wrong thing by returning zero when basis for $direction is zero, thereby setting $delta_to_add to zero as well.

I used to think there was a function that would return -1 for values less than 0, and 1 otherwise, so that you could simply use the multiplication operator to copy the sign of one variable onto another. I also dimly recall at least one discussion about this at the Monastery, but now I can't seem to locate either such a function or the previous debate. Oh well...

(updated slightly in hopes of making the 2nd paragraph clearer)