#!/usr/bin/perl --
use strict;
use warnings;
use Data::Dump qw/ pp /;
use Tk;
my $mw = tkinit;
$mw->Entry(
    -validate => 'all',
    -validatecommand => sub { 
        warn pp(\@_);
        return 1 if !length$_[0]
                    or $_[0] =~ m{^\d+$};
        return undef;
    },
)->pack;
$mw->MainLoop;