Quoting perlop:
Note that just as in C, Perl doesn’t define when the variable is incremented or decremented. You just know it will be done sometime before or after the value is returned. This also means that modifying +a variable twice in the same statement will lead to undefined behaviour. Avoid statements like: $i = $i ++; print ++ $i + $i ++; Perl will not guarantee what the result of the above statements is.
To me, that seems that perlop quite explicitly says the behaviour is undefined.

Perhaps you have an other reading of perlop which defines the order?

In reply to Re^6: Boolean counter? by JavaFan
in thread Boolean counter? by DreamT

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