Firstly, when posting example code, please use "code" tags, as described in Writeup Formatting Tips. This makes it easier for others to understand and download your code.

Secondly, it would have been better to create a more meaningful title for the posting: "Simple or not" conveys no useful meaning.

Now, on to (what I think is) your problem. If you are trying to replace all whitespace in your input with tabs, then preserve the newline, the following should accomplish that task (untested):

use strict;
use warnings;

open my $fh_in , '<', 'test1.txt' or die "Can not open file test1.txt:
+ $!";
open my $fh_out, '>', 'test2.txt' or die "Can not open file test2.txt:
+ $!";
while (<$fh_in>) {
    s/\s+/\t/g;    
    print $fh_out "$_\n";
}
close $fh_out;
close $fh_in;
[download]

In my opinion, you never clearly stated what your goal was. It would have been more useful if you had supplied a few sample lines of your input file, your actual output and your expected output.