$a = $b = $c = 1; 
print $a++, " ", $a++, "\n";
print $b++, " ", $b, "\n";
print $c, " ", $c++, "\n";
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Isn't order of operations fun? I'm not sure I completely understand this either. As far as I can tell, although addition is generally left-associative, all operations in the equation (++ operators) are run first. The auto-increment operators have higher precedence than the addition operator. The first two equations in your example are easy to understand. The first $a++ returns 1, which is added to the second $a++ which returns 2, since the inital ++ has already incremented the variable. The $b equation is much the same, since the post-increment operator is superfluous. The final equation is tricky. Because of precedence, the auto-increment runs first, setting the value of the rhs of the equation to 1, and setting $c to 2, which makes that the lhs. Keep in mind, that the addition operator operates on the values returned by the auto-increment operators at the time that they ran.

Edit: As Abigail-II pointed out, I have no idea what I'm talking about in terms of standards. I am curious, though, why the standards-maintainers chose to keep this behavior undefined.