G'day cniggeler,

"... state of the Entry ... is actually an array ..."

Your underlying problem is that you're calling configure() where you should be calling cget(). Take a look at Tk::options; from the SYNOPSIS:

$value = $widget->cget('-option');
...
@list = $widget->configure('-option');
[download]

That's why you're getting an array.

There's a lot of other problems with the code you presented. I'd start by putting the following before the code you have there and fixing all of the issues it tells you about. See "perlintro: Perl introduction for beginners" if this code is unfamiliar to you.

use strict;
use warnings;
[download]

You should also remove leading '&'s from subroutine calls unless you know exactly what you're doing and can explain fully why you're doing it. See "perlsub: Perl subroutines".

"... a checkbox is checked elsewhere ..."

Your code doesn't contain any references to Tk::Checkbutton. There could be some other problem(s) there but, without seeing the code, it's impossible to tell.