This appears to do what you want:

#!/usr/bin/perl

use strict;
use warnings;
use Tk;

my $t1;
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 => 'normal');

    if ( !Exists( $t1 ) ) {
        $t1 = $mw->Toplevel();
            sub  { $t1->withdraw;
                  $spawn_button->configure(-state => 'disabled');
        $t1->geometry('300x100+100+100');
        $t1->title( "Toplevel" );
        $t1->Button(
            -text    => "Close",
            -command => sub { $t1->withdraw;
                 $spawn_button->configure(-state => 'normal');
                 }
        )->pack;
      }
    } else {
        $t1->deiconify();
        $t1->raise();
    }
}
[download]

update: Strange, but the script on my computer worked, but after downloading the script from perlmonks---it doesn't work. Sorry...

update: Fixed, it should work now.