erez_ez,
You really should use warnings. What I have does what you want, but is really a bad way of doing it - you should look at the advice from zentara. I only had a couple of minutes in between playing blocks with my 2 year old.

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

my $main = MainWindow->new;

$main->minsize(qw(350 350));
$main->title("GUI");
$main->configure();
    
my $l_frame = $main->Frame()->pack(-side => 'left');  
          
my ($checked, $entry);
my $check_box = $l_frame->Checkbutton(
    -variable => \$checked,
    -command => sub {
        open_menu($checked, \$entry);
    }
)->pack(); 
    
MainLoop; 

sub open_menu {
    my ($checked, $entry) = @_;
    if ($checked) {
        $$entry = $l_frame->Label(
            -text       => 'enter your name: ',
            -background => 'white'
        ) if ! defined $$entry;
        $$entry->pack();
    }
    else {
        $$entry->destroy;
        $$entry = undef;
        $main->update;
    }
}
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