#!/usr/bin/perl -w
use strict;
use warnings;

my $file = 'hugefiletobereversed';
open FH, 'hugefiletobereversed';
while (our $newline = <FH>)
{
{
   local @ARGV = ($file);
   local $^I = '.bac';
   while(<>){
      if ($. == 1) {
         print "$newline\n";
      }
      else {
         print;
      }
   }
}
}