If the toplevel hasn't been mapped yet or has been withdrawn you'll want to invoke the deiconfiy method to make sure the window is mapped in de-iconified form, and then the raise method to bring the window to the top of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;
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