s/(".*?")/($x=$1)=~y: :_:, $x/ge)

[CUT]

  =~y: :_:,                '=~y: :_:, '
----------------------------------------------------------------------
  $                        before an optional \n, and the end of the
                           string
----------------------------------------------------------------------
  x/ge                     'x/ge'

##</code><code>##

s/(".*?")/($x=$1)=~y: :_:, $x/ge)
^^^^^^^^^1
          ^^^^^^^2
                  ^^^^^^^3
                          ^^4
                             ^^^5