Hope this demo can help you to understand:

use Tk;
use strict;

my $val;

my $mw = new MainWindow;
$mw->Checkbutton(onvalue => "I am selected",
                 offvalue => "I am deselected",
                 variable => \$val)
   ->pack;
$mw->Entry(textvariable => \$val)->pack;

MainLoop;
[download]

In a real situation, more likely, you would do this:

$mw->Checkbutton(onvalue => 1,
                 offvalue => 0,
                 variable => \$val);
[download]

To check whether the Checkbutton is selected, just check whether $val is true, do something like:

if ($val) {
     ...
} else {
     ...
}
[download]