The problem is that your overload function does not take into account the differences when being called as a conventional method, and when it is called via the overloading. In the latter case, there is an extra 3rd parameter identifying the ordering of the operands for the operator.

From the overload pod:

Calling Conventions for Binary Operations

The functions specified in the use overload ... directive
 are called with three (in one particular case with four,
 see 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.
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