#!/usr/bin/perl

use warnings;
use strict;

open( my $output, '>', '/tmp/output.txt' )
  or die "Error: Cannot open the file - $!\n";

open( my $input, '<', '/tmp/input.txt' )
  or die "Error: Cannot open the  file - $!\n";

print {$output} while (<$input>);   ## This is were the problem is.

close( $input );
close( $output );