Re^4: Why does the first $c evaluate to the incremented value in [$c, $c += $_] ?by smls (Friar)
|on Mar 05, 2014 at 00:34 UTC||Need Help??|
D'oh... Yeah, you're right.
So, for the record, the evaluation order is in fact well-defined: It strictly evaluates higher-precedence operators first, before evaluating any operands of any less deeply nested part of the expression.
That's the source of my confusion: I intuitively expected Perl to only use the operator precedence information to implicitly "add parenthesis" for disambiguation, but then evaluate the expression left-to-right, only recursing into nested sub-expression once they are encountered.
That's two conceptually very different ways of defining evaluation order, but easy to miss because in practice it probably never makes a difference except in special cases involving += and friends.
Well, almost never... :) Here's a demonstration of the evaluation order using tied scalars that report when they're being accessed:
As for the mentioned perlop paragraph re. "undefined behavior", I suppose that refers to what Eily says here.