no one mentioned it yet, but in case you don't really need a binary operator, you can go for a normal function, because arguments are "passed by alias" in Perl.

  DB<11> sub op { $_[0] += $_[1] }

  DB<12> $x=4

  DB<13> op($x,5)

  DB<14> p $x
9
  DB<15>
[download]

as you can see, you can change the states of passed variables.

That's normally the most maintainable approach, especially when combined with a good name for the operation.