DB<171> $str=$str0
 => " bla0 \"ignore a1\" bla1 \"ignore a2\" bla2"

  DB<172> @parts=split '"',$str
 => (" bla0 ", "ignore a1", " bla1 ", "ignore a2", " bla2")

  DB<173> map {s/a/A/ unless $x++%2} @parts
 => (1, 1, 1, 1, 1)

  DB<174> $str=join '"',@parts
 => " blA0 \"ignore a1\" blA1 \"ignore a2\" blA2"