A quick demo:

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");
[download]

This prints:

abcdefghijklmnopqrstuvwxyz
ABCDEFGHIJKLMNOPQRSTUVWXYZ
[download]