:D:D:D:D:D:D:D:D:D

http://docs.activestate.com/activetcl/8.5/tcl/TkCmd/pack.htm

You're calling configure on $ent but $ent is not an object

$ent is assigned the return value from g_pack

g_pack doesn't return an object

the return value of the g_pack method is not an object

g_pack returned something which is not an object

g_pack returned the empty string ""

"" the empty string does not have a configure method

"" the empty string is not an object

you can't call methods on the empty string ""

:D:D:D:D:D:D:D:D:D

Thank you Anonymous Monk. I fixed the script but the validatecommand is still not doing what I expect. I want it to give an immediate error if an invalid character is input into the entry.

#! perl
use strict;
use warnings;
use Tkx;

my $value;
my $mw  = Tkx::widget->new(".");
my $frame = $mw -> new_frame();
$frame->g_pack();
my $ent = $frame ->new_entry(-textvariable => \$value);
$ent->g_pack();
$ent->configure(-validatecommand => [sub { $_[0] =~ /^(?:|-|\d+|-\d+)$
+/ }, Tkx::Ev('%P'), $ent], -invalidcommand  => \&lam_num_error);

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

my $reset_frm = $mw->new_frame();
$reset_frm->g_pack(-fill => 'both');
my $reset_button = $reset_frm->new_button(-text    => "Reset",
                                      -command => \&do_reset,
                                      -font    => "ansi 10 bold")->g_p
+ack();
Tkx::MainLoop();

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

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

sub lam_num_error
{
    Tkx::tk___mmessageBox(-message => "The input must be an integer.")
+;
}

__END__
[download]

Thank you Anonymous Monk. I fixed the script but the validatecommand is still not doing what I expect. I want it to give an immediate error if an invalid character is input into the entry.

Well, fixing g_pack so you have $ent is not the only change you made

If you employ basic debugging checklist, I'm sure you'll notice what isn't getting called and when