We can clearly see 'undef' is being passed and that's what's throwing off your conditional. This is so counter-intuitive that I assume I'm missing something, though a quick scan through the docs doesn't reveal what that would be.

The relevant part of the documentation is:

       Calling Conventions for Binary Operations

       The functions specified in the "use overload ..." direc­
       tive 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 infor­
       mation by examining the third argument, which can take
       three different values:

       FALSE  the order of arguments is as in the current opera­
              tion.

       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 "Calling
              Conventions for Mutators".
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The first argument is the object, no surprise here. Since the stringify operation is a unary operation, the second argument is undefined. The third argument is the empty string, which is false, but defined. And this fits the description of the documentation - the arguments aren't reversed, nor is the operation an assignment variant.

Abigail