This is Anon-a-monk #2 chiming in here, trying to get us back on the same track. The problem of EOD arises whenever side-effects (such as assignment) occur inside your expression. If you eliminate all assignments (read: side-effects) in your expression, you'll be guarenteed not to have an evaluation order dependency. Here's a quick rule-of-thumb: If you are using assignment, make sure the RHS doesn't contain terms that cause an assigment to the same variable on the LHS. Examples... 
$i = 4+$j;   #good, no assignments on RHS
$i = 4+$j++; #also good, assignment ($j++) doesn't refer to LHS ($i)
$i = $i + 1; #good, still no assignments on RHS
$i = $i++    #BAD, assignment ($i++) uses variable which occurs on LHS

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For further reading, I recommend Section 3.1.3 of SICP.

In reply to Re^15: Why is the execution order of subexpressions undefined? by Anonymous Monk
in thread Why is the execution order of subexpressions undefined? by BrowserUk

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