I didn't like waitWindow() because it will stop all program execution after that point. The code below works and seems to be a better solution (and is non-OS Specific) (Note, toplevel window(s) do continue after waitWindow() and can call and run subroutines(callbacks), In the case I needed to get around I needed code to run after the waitWindow() )

use Tk;
use strict;

my $tl; #<- Make this avaliable for testing globally.
my $mw = MainWindow->new;

$mw->title("MainWindow");
$mw->Button(-text => "Toplevel", -command => \&do_Toplevel)->pack( );
$mw->Button(-text => "test Toplevel", -command => \&test_toplevel)->pa
+ck( );

MainLoop;
sub do_Toplevel {
  if (! Exists($tl)) {
    $tl = $mw->Toplevel( );
    $tl->title("Toplevel");
    $tl->Button(-text => "Close", 
                -command => sub { $tl->withdraw })->pack;
  } else {
    $tl->deiconify( );
    $tl->raise( );
  }
}

sub test_toplevel {
    if ( Exists($tl)) {
    print "Top level is here\n";
    } else {
    print "Top level is gone\n";
    }
}
[download]