Maybe I'm misinterpreting what you want to do, maybe you just want to call $toplevel->raise if it isn't on top? You can test for window visibility, and raise it if it isn't visible.

#!/usr/bin/perl
use Tk;

$mw = tkinit;
$t = $mw->Toplevel;
$t->withdraw;
$t->Label(-text => "Testing...")->pack;
$t->Button(
   -text => "Withdraw",
   -command => sub {$t->withdraw},
)->pack;
$t ->overrideredirect(1); #to top on all virtual desktops 

$mw->Button(
   -text => 'Test',
   -command => sub {
       $_->deiconify, $_->raise for $t;
   },
)->pack;

MainLoop;
[download]

Also you might want to check out the Tk::Wm perldoc. The following dosn't work on fvwm2, but is reported to work on KDE.

#from comp.lang.perl.tk
#The following works, at least with KDE 2. I would be glad to #here su
+ccess reports for KDE 3.
#Regards, Slaven  Rezic

use Tk;
$top = new MainWindow;
keep_on_top($top);
MainLoop;

sub keep_on_top {
    my $w = shift;
    my($wrapper) = $w->toplevel->wrapper;
    $w->property('set', '_NET_WM_STATE', "ATOM", 32,
         ["_NET_WM_STATE_STAYS_ON_TOP"], $wrapper);
}

__END__
[download]

Here's a modified version which first checks whether the window manager supports the _NET_WM_STATE_STAYS_ON_TOP atom.

sub keep_on_top {
    my $w = shift;
    my($wrapper) = $w->toplevel->wrapper;
    eval {
        if (!grep { $_ eq '_NET_WM_STATE_STAYS_ON_TOP' } $w->property(
+'get', '_NET_SUPPORTED', 'root')) {
            die "_NET_WM_STATE_STAYS_ON_TOP not supported";
        }
        $w->property('set', '_NET_WM_STATE', "ATOM", 32,
                     ["_NET_WM_STATE_STAYS_ON_TOP"], $wrapper);
    };
    if ($@) {
        warn $@;
        0;
    } else {
        1;
    }
}
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The code could be expanded to use the <Visibility> or overrideredirect hacks on other window managers.