#!/usr/bin/env perl

use strict;
use warnings;

use Tk;

my $mw = MainWindow->new();

$mw->Label(-text => 'Enter integers only:')->pack;
$mw->Entry(
    -validate => 'key',
    -validatecommand => sub { $_[0] =~ /^\d+$/ },
    -invalidcommand => \&lam_num_error,
)->pack;

$mw->Button(-text => 'Exit', -command => sub { exit })->pack;

MainLoop;

sub lam_num_error {
    $mw->messageBox(-message=>"The input must be an integer.");
}