$txtbox = $textParent->Text(...)->pack();
...
$cbtnParent->Checkbutton(-variable => \$txtboxColor,
                         -text => 'Set textbox color',
                         -offvalue => 'gray',
                         -onvalue => 'white',
                         -command => sub {$txtbox->configure(-background => $txtboxColor)}
                         )->pack(); # (or whatever geometry mgmt you're using)