found solution myself for my second question. OnDestroy will do the trick.. Thanks, perlmonks is a very helpful place.

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

my $tl;
my $mw = MainWindow->new;
$mw->title( "MainWindow" );

my $spawn_button = $mw->Button( -text => "Toplevel", 
                -command => \&do_Toplevel )->pack();

MainLoop;

sub do_Toplevel {
        $spawn_button->configure(-state=>'disabled');

    if ( !Exists( $tl ) ) {
        $tl = $mw->Toplevel();
        $tl ->OnDestroy(  
            sub { $tl->withdraw; 
                 $spawn_button->configure(-state=>'normal');
             }
        );
        $tl->geometry('300x100+100+100');
        $tl->title( "Toplevel" );
        $tl->Button(
            -text    => "Close",
            -command => sub { $tl->withdraw; 
                 $spawn_button->configure(-state=>'normal');
                 }
        )->pack;
    }
    else {
        $tl->deiconify();
        $tl->raise();
    }
}
===============
[download]