#!/usr/bin/perl
use strict;

open(A, ">A.TXT") or die $!;
print A "A\n";
close A or die $!;

open(A, ">a.txt") or die $!;
print A "a\n";
close A or die $!;

open(A, "A.TXT") or die $!;
while (<A>) { print }
close A or die $!