You might consider using something like Getopt::Long:

#!/usr/bin/perl -w

use strict;

use Getopt::Long;

my ($username, $password);

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

Now you can do this:

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

As a note, putting the password in the command line on a system which is not 100% your own is a really bad idea since this password will show up in the process listing table. Some programs actually mangle the $0 variable to hide this:

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

But there still is a narrow window of opportunity where they might see it.