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

my $mw = MainWindow->new(
    -title=>"Foo!",
);

### '-container' property needs to be true
my $f  = $mw->Frame(
    -width=>400,
    -height=>400,
    -container => 1,
)->pack;

$\ = "\n";

print "Tk is $Tk::VERSION; Window id is ",
      $f->winfo('id');

### add the following line, change the -use line below
my $winfo = hex $f->winfo('id');
my $child = $mw->Toplevel(
    -title => "Bar!",
    -use => $winfo,
);

$child->Label(
    -text=>"Bar!",
)->pack;

MainLoop();
__END__
output:
Tk is 800.023; Window id is 0xff0344