## START PSEUDO-CODE
my $string = $big_string_from_my_original_example;
my @array_of_stuff = split
  /\b(?:STARTPRESERVE)|(?:STOPPRESERVE)\b|
  \b(?:MARKA)|(?:MARKB)\b/, $string);
foreach (@array_of_stuff) {
  if (&check_for_marker) {
    push @new_array, $_;
    next;
  }
  s/\s+/ /g;
  push @new_array, $_;
}
$string = join '', @new_array;
## END PSEUDO-CODE