I think that the overloading procedure views '+=' as an assignment operation ($x = $x + $addon), not as an inplace mutation ($x += $addon).
The former requires a value to be returned; the latter does not.
At least, it's behaving as though that's the case.

That's why that documentation snippet I posted goes on to say: 
         The subroutine for the assignment variant of an operator is
         required only to return the result of the operation. It is
         permitted to change the value of its operand (this is safe be
+cause
         Perl calls the copy constructor first), but this is optional 
+since
         Perl assigns the returned value to the left-hand operand anyw
+ay.

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Note that it's specifically stating that it's "permitted" (but not mandatory) "to change the value of its operand ".
If it was being deemed to be an inplace mutation, then it would have to change the value of its operand.

AFAICS, there aren't many modules that overload '+=', but Math::BigFloat does: 
  '+='    =>      sub { $_[0] -> badd($_[1]); },

[download]
which I manually altered to: 
  '+='    =>      sub { $_[0] -> badd($_[1]); 42 },

[download]
I then get: 
D:\>perl -MDevel::Peek -MMath::BigFloat -le "$x = Math::BigFloat->new(
+20); $x += 100; print $x; Dump $x;"
42
SV = PVIV(0x1bc99137430) at 0x1bc9913a450
  REFCNT = 1
  FLAGS = (IOK,pIOK,pPOK)
  IV = 42
  PV = 0x1bc9b44fbe0 "42"\0
  CUR = 2
  LEN = 16

[download]
In that pure perl environment it's clearly the returned result, not the result of the inplace addition, that's being seen.

I'm now feeling more confident that the need for '+=' overloading to return the new value is intended (though unexpected) behaviour.

Cheers,
Rob

In reply to Re^9: Puzzled by value of $overload::ops{binary} by syphilis
in thread Puzzled by value of $overload::ops{binary} by syphilis

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