#! perl
use strict;
use warnings;
use Tk;

my $value;
my $mw  = new MainWindow;
my $ent = $mw->Entry(-textvariable    => \$value,
                     -validate        => 'key',
                     -validatecommand => sub { $_[0] =~ /^(?:|-|\d+|-\d+)$/ },
                     -invalidcommand  => \&lam_num_error)->pack();

my $print_button = $mw->Button(-text    => "Print",
                               -command => \&printx,
                               -font    => "ansi 10 bold")->pack();

my $reset_frm = $mw->Frame();
$reset_frm->pack(-fill => 'both');
my $reset_button = $reset_frm->Button(-text    => "Reset",
                                      -command => \&do_reset,
                                      -font    => "ansi 10 bold")->pack();
MainLoop;

sub printx
{
    print $value unless $value eq '-';
}

sub do_reset
{
    $ent->delete(0, 'end');
}

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

__END__