Jonathan, I think what you want is something like this:
(make sure data.txt is in the local directory, or specify the full path)

use strict;
use LWP::Useragent;
use HTTP::Request::Common;

my $ua = new LWP::UserAgent;

my $res = $ua->request(POST 'https://<address>/cgi-bin/upload.pl',
     Content_Type => 'form-data',
     Content      => [ foo    => 'bar',
                       bar    => 'baz',
                       FILE1  => ["data.txt"],
                     ]
);
[download]

update: Changed file field name.