When you declare the Checkbutton, you would want to have the handle for the text box to be in scope, so you can refer to that in a sub, to be assigned to the Checkbutton's "-command" attribute -- something like the following:

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