#!/usr/bin/perl
use Tk;

my($text1, $text2);
my $max_len = 8;

my $mw = tkinit;

my $entry1 = $mw->Entry(-textvariable => \$text1)->pack();
my $entry2 = $mw->Entry(-textvariable => \$text2)->pack();

$entry1->bind('<Key>' => sub { $text1 = substr($text1, 0, $max_len);
                               $entry2->focus() if length($text1) == $max_len;
                               });
$entry2->bind('<Key>' => sub { $text2 = substr($text2, 0, $max_len); });

MainLoop();