use Win32::API;
Win32::API->Import('user32', 'FindWindowA', 'PP', 'N');

my $title = 'Title/Window Caption of StatusBar Window';

if (FindWindowA(0, $title)) {
  # window exists, do status bar update
}
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Of course, that is Win32-specific. I couldn't help posting that. I've been fiddling with Win32's API lately and find it quite interesting (if at times frustrating because of Win32::API::Struct and other things). I haven't used Tk as of yet, so I can't be certain of this, but can you not keep a $status_open variable and hook into the event of a closed window and set the variable appropriately. Thus, before you update the status, check the variable to ensure the window is open. Doesn't stop the user from closing the window right in the middle of your update though. Why not create a status window the user cannot close (surely Tk allows you to create windows without minimize, maximize, closing functionality).

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";
    }
}
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#!/usr/bin/perl
use Tk;
$mw = MainWindow->new;
$mw->title("MainWindow");
$mw->Button(-text => "Toplevel", -command => \&do_Toplevel)->pack(  );

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(  );
  }
}
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Here is an example which signals the MainWindow if a toplevel window is closed. I'm sure you can find a way to work it in to your script.

#!/usr/bin/perl -w
# waitwindow.pl 
use Tk;
use strict;

my $mw = MainWindow->new();
my $toplevel_count = 0;
my $last_window_died = 1;

$mw->Checkbutton(
    -text => 'No toplevel windows alive',
    -variable => \$last_window_died,
)->pack();

$mw->Button(
    -text => 'Create a toplevel',
    -command => sub {
        my $toplevel = $mw->Toplevel;
        $toplevel_count++;
        $last_window_died = 0;
        $toplevel->Button(
            -text => 'Destroy Me',
            -command => sub {$toplevel->destroy}
        )->pack();
        $toplevel->waitWindow;
        #main window keeps processing events... 
        #this code is executed after $toplevel is destroyed 
        $toplevel_count--;
        if ($toplevel_count == 0) {
            #toggle the checkbutton 
            $last_window_died = 1;
        }

    },#end sub 
)->pack(
    -fill => 'x',
    -expand => 1,
);

$mw->Button(
    -text => 'Quit',
    -command => sub {$mw->destroy},
)->pack(
    -fill => 'x',
    -expand => 1,
);
MainLoop();
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