I believe I understand your question. (disclaimer: this in no way is an admission of liability if I do not, in fact, understand your question)

What I think you should do (for simplicity's sake) is run 2 passes on the file, like so:

#use whateveryouwant;
my $filename = "insertfilenamehere";
open INPUT, $filename or die;
my $modify_state = "false";
my $line = "";
while ($line = <INPUT>)
{
  chomp($line);
  #look for the value
  if ($line =~ /are these the droids you're looking for?/)
  {
     $modify_state = "true";
     last;
  }
}
close INPUT;
if ($modify_state eq "false")
{
  open INPUT, $filename;
  open OUTPUT, ">output" or die;
  while ($line = <INPUT>)
  {
     #modify the line here

     #and print to the output file 
  }
  close OUTPUT;
  close INPUT;
  #rename the output file back to the original filename.
}
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Notes about the code: It's not particularly efficient. You might have to go through the file twice (worst case). However to me the advantage is that it is simple. You look for the value. if you find it then stop looking and start over modifying the file. If you dont find it then you are already done. Hope this helps.

Final note. This code has not been tested and it will take a little work to address your problem. It is intended as a framework for you to use.