#!/usr/bin/perl -w

use strict;

use Getopt::Long;

my ($username, $password);

my $result = GetOptions("username=s" => \$username,
                        "password=s" => \$password);

##</code><code>##

% myscript --username=foo --password=bar

##</code><code>##

$0 =~ s/password=\S+/password=xxx/;