Completely untested, but my interpretation of the docs suggests it should look something like this:

use LWP::UserAgent;
use HTTP::Request::Common;
use Digest::MD5::File qw(file_md5_hex);
use File::Slurp;

#text file to upload
my $text_file = 'file.txt';

#calulate Content-MD5 Header
my $md5hash = file_md5_hex($text_file);

#read file
my $file_contents = read_file($text_file);

#fields to post
my %post_parameters = (
    'a' => 'red',
    'b' => 'blue',
    'c' => 'green'
);

$ua = LWP::UserAgent->new();
$ua->agent('Blah/Perl');
$ua->post( 'http://amazonmws.blah', 
    Content_Type =>'form-data',
    Content-MD5  => $md5hash,
    Content      => [
        file => [ $file_contents, $text_file ],
        %post_parameters 
    ],
);

print STDERR $rsp->status_line;
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