Just one word: side effects. (No, that's two words. Whatever. :))

If your expression contains parts with side effects (changes of "global" variables, reading input from the outside world), then it may depend on execution order. Actually, it only will, if you do read from it more than once. So,

(scalar <FH>, scalar <FH>)
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depends on execution order, as does

($i++, $i++)
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while

(scalar <FH>, $i++)
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or

($i++, $k++)
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do not.

Expressions with no side effects don't ever depend on execution order. That's one of the basic ideas behind functional programming.

update: It's not always obvious to see when a statement has side effects. Two examples I just recently thought of are rand and the operators .. and ..., which all have a built-in variable holding their current state.