use strict;
use warnings;
open (FF,"filename.txt"); # assume file is having 6 lines
@array=<FF>;
open (FF1,">file2write.txt");

for ($i=0;$i<=6;$i++)
{

  if($i%2==0)
 {
   print FF1, @array[$i+1];
}

if ($i%2==1)
{
 print FF1,@array[$i-1];
}