The regular expression:

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

matches as follows:

NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  s/                       's/'
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    "                        '"'
----------------------------------------------------------------------
    .*?                      any character except \n (0 or more times
                             (matching the least amount possible))
----------------------------------------------------------------------
    "                        '"'
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
  /                        '/'
----------------------------------------------------------------------
  (                        group and capture to \2:
----------------------------------------------------------------------
    $                        before an optional \n, and the end of
                             the string
----------------------------------------------------------------------
    x=                       'x='
----------------------------------------------------------------------
    $                        before an optional \n, and the end of
                             the string
----------------------------------------------------------------------
    1                        '1'
----------------------------------------------------------------------
  )                        end of \2
----------------------------------------------------------------------
  =~y: :_:,                '=~y: :_:, '
----------------------------------------------------------------------
  $                        before an optional \n, and the end of the
                           string
----------------------------------------------------------------------
  x/ge                     'x/ge'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------