DB<185> $x=0;$str=$str0
 => " bla0 \"ignore a1\" bla1 \"ignore a2\" bla2"

  DB<186> $str= join '"', map {s/a/A/ unless $x++%2;$_} split '"', $str
 => " blA0 \"ignore a1\" blA1 \"ignore a2\" blA2"