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 because
         Perl calls the copy constructor first), but this is optional since
         Perl assigns the returned value to the left-hand operand anyway.

##</code><code>##

  '+='    =>      sub { $_[0] -> badd($_[1]); },

##</code><code>##

  '+='    =>      sub { $_[0] -> badd($_[1]); 42 },

##</code><code>##

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