use Fcntl 'SEEK_CUR';
use strict;
use warnings;

open FILE, ">chars";
for (0 .. 25) {
	print FILE chr(ord('a') + $_);
}
close FILE;
system("type chars");
print "\n";

open(FILE,"+<chars");
while (1) {
	sysread(FILE, my $chr, 1);
	last if (!ord($chr));
	sysseek(FILE, -1, SEEK_CUR);
	syswrite(FILE, chr(ord($chr) - ord('a') + ord('A')));
}
close FILE;
system("type chars");