i think you can use in-place edit to add lines anywhere in the file provided you know the line number for addition.
the below code will add Hello in first line of file.

#!/usr/bin/perl
use warnings;
use strict;
my $string="Hello";

{
   local @ARGV = ("$ARGV[0]");
   local $^I = '.bac';
   while(<>){
      if ($. == 1) {            # if line no is 1
         print "$string$/";   
         print;                 # Also print the current line
      }
      else {
         print;
      }
   }
}
[download]

if you name it edit.pl , then calling edit.pl <filename> will do the job.
you can customize other stuff in the program, i just expressed idea.
may be Monks can comment if its the right way or not, iam still a beginner and learner