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;