+ skript.pl
+ perllib
     + Tk
         + UserPassword.pm

#!/usr/bin/perl

use strict;
use warnings;
use Tk;
use lib qw(./perllib);
use Tk::UserPassword;

my $mw = tkinit();
# der andere Tk-Kram
$mw->Button( -command => \&irgendein_callback, -text => 'Klick mich' )->pack;
MainLoop;

sub irgendein_callback{
    my $up = Tk::UserPassword->new($mw);
    my ($user,$pass) = $up->Show();
    print $user, " :: ", $pass;
}