#!/usr/bin/perl
use warnings;
use strict;

use Tk;

my ( $entry, $cbvalue );

my $mw = tkinit;

$mw->Checkbutton(
    -text     => 'Use Entry widget below?',
    -variable => \$cbvalue,
    -command  => \&SetState,
)->pack;

$entry = $mw->Entry->pack;
SetState();
MainLoop;

sub SetState {
    $entry->configure( -state => $cbvalue ? 'normal' : 'disabled' );
}