use strict;
use warnings;
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";

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

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