my @file;
my $changed;
{
   open(my $fh, '<', $fn)
      or die("Unable to open file \"$fn\": $!\n");

   while (<$fh>) {
      if (...[ need to delete this line ]...) {
         $changed = 1;
      } else {
         push @file, $_;
      }
      if (...[ need to insert a line after this line ]...) {
         push @file, ...;
         $changed = 1;
      }
   }
}

if ($changed) {
   open(my $fh, '>', $fn)
      or die("Unable to open file \"$fn\": $!\n");

   print $fh @file;
}