The C standard says:

Between the previous and next sequence point a scalar object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value shall be accessed only to determine the value to be stored.

The == operator doesn't introduce a sequence point. You're trying to access the prior value of the variable to 1) determine the value to be stored 2) determine the value to be compared with whatever increment operator yields. The C standard calls that 'undefined behavior', which means anything can happen.

perlop says:

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.