Let's say fruit.txt contains the following.

apple
banana
orange
pineapple

To determine if fruit.txt does or does not contain a certain type of fruit, you can first store the fruit.txt in an array. In this example, the content of the fruit.txt file is stored in the @fruit array.

my $file = "fruit.txt";

# Open the file for reading
if (open FH, "<", $file) {

  # Load the content of the file into an array
  my @fruit = <IN>;

  # Close the file
  close IN;
}
else {
  print "failed to open $file \n";
  exit 1;
}

You can then use grep to determine if the @fruit array does or does not contain a certain type of fruit.

if (grep(/apple/, @fruit)) {
  print "fruit.txt contains 'apple' \n";
}

if (not grep(/peach/, @fruit)) {
  print "fruit.txt does not contains 'peach' \n"; 
}

Let's say you want to append fruit to fruit.txt only when fruit.txt does not contain the fruit in question.

if (not grep(/peach/, @fruit)) {
  print "fruit.txt does not contains 'peach' \n"; 

  # Open the file for appending
  open(OUT, ">>", $file) or die "Cannot open $file $! \n";

  # Append 'peach' to fruit.txt
  print OUT "peach \n";

  # close the file
  close OUT;
}