Actually, the third argument doesn't indicate whether the statement is in void context, but rather whether the statement is an assignment varient. Here's the relevant section from perldoc overload:

Calling Conventions for Binary Operations

The functions specified in the use overload ... directive are called with three (in one particular case with four, see the section on Last Resort) arguments. If the corresponding operation is binary, then the first two arguments are the two arguments of the operation. However, due to general object calling conventions, the first argument should always be an object in the package, so in the situation of 7+$a, the order of the arguments is interchanged. It probably does not matter when implementing the addition method, but whether the arguments are reversed is vital to the subtraction method. The method can query this information by examining the third argument, which can take three different values:

FALSE	the order of arguments is as in the current operation.
TRUE	the arguments are reversed.
undef	the current operation is an assignment variant (as in $a+=7), but the usual function is called instead. This additional information can be used to generate some optimizations. Compare the section on Calling Conventions for Mutators.

The point is so that you can distinguish between

print($obj += 6);
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and

print($obj + 6);
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They should both print the same thing, but in the first case $obj should be increased by 6 and in the second case it shouldn't.

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