#!/usr/local/bin/perl -w
use Tk;
use strict;

my $str = "AaBbCcDdEeFfGgHhIiJjKkLlMmNnOoPpQqRrSsTtUuVvWwXxYyZz0123456789";

my $mw = MainWindow->new;
my $lframe = $mw->Frame->pack(-fill => 'both', -side => 'left', -expand => 1);

my $entry = $mw->Entry(
    -textvariable => \$str,
    -width => 12, 
    -font => "{Comic Sans MS} 72",
    -relief => 'raised',
    -highlightthickness => 0,
)->pack(-expand => 1, -fill => 'x', -side => 'left');


my $repeat_id = $mw->repeat(300, \&shift_banner);

MainLoop;

sub shift_banner {
    my $newstr = substr($str, 1) . substr($str, 0, 1);
    $str = $newstr;
}