#!/usr/bin/perl -l

use strict;
use warnings;

my $len=80;
my $line = '!' x $len;
substr +($line ^= $line), $len/2, 1, "\xFF";

{
    local $_=$line;
    y/\0\xFF/ ^/;
    print;
    my ($lf, $rt) = map {
        substr $line x 2, $_ % $len, $len;
    } 1, -1;
    $line=$lf ^ $rt;
    select undef, undef, undef, .1;
    redo;
}

__END__