# $str =~  s/(?<!bar)(.*?)foo(?!.*?qux)/$1.'123'/eg;   # one of several failed attempts

$str =
  join '',                               # glue back together
    map { s/(?<!bar)(.*?)foo(?!.*?qux)/$1 .'123'/eg; $_ }  # replace on the non- bar-foo-qux elements.
    split /(bar.*?foo.*?qux)/, $str;     # get elements that are either the bar-foo-qux form or not.