If the toplevel hasn't been mapped yet or has been withdrawn you'll want to invoke the deiconfiy method to make sure the window is mapped in de-iconified form, and then the raise method to bring the window to the top of the stacking order. The Tk::Widget and Tk::Wm man pages should be helpful.

This quickie demo should give you a good start:

use Tk;
$mw = tkinit;
$t = $mw->Toplevel;                           
$t->withdraw;
$t->Label(-text => "Testing...")->pack;
$t->Button(
    -text => "Withdraw",
    -command => sub {$t->withdraw},
)->pack;
$mw->Button(
    -text => 'Test',
    -command => sub {
        $_->deiconify, $_->raise for $t;
    },
)->pack;                    
MainLoop;
[download]

This is a bit of a guess, but the Tk::Wm POD mentions a focus method.

Maybe $toplevel->focus will work?

From the Tk::focus POD:

       $widget->focusForce
           Sets the focus of $widget's display to $widget, even if the
+ appli-
           cation doesn't currently have the input focus for the displ
+ay.
           This command should be used sparingly, if at all.  In norma
+l usage,
           an application should not claim the focus for itself;  inst
+ead, it
           should wait for the window manager to give it the focus.
[download]

I've found only one case where focusForce was required to get focus back to one of my windows, and that was after a call to getOpenFile from a composite widget. I'm still not sure if focusForce was required because of something I was doing wrong or a focus bug introduced by getOpenFile (I suspect the former).

If raising a window doesn't cause the window manager to give it focus, then $widget->focus should be sufficient.