If the toplevel hasn't been mapped yet or has been withdrawn you'll want to invoke the deiconfiy method to make sure the window is mapped in de-iconified form, and then the raise method to bring the window to the top of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