i got this to work just fine on one of my image-uploading galleries. i think you've simply formatted your data structure incorrectly. the documentation is a bit confusing as it mentions the structure you use and then the one i used. nonetheless, seems to work.

#!perl -wl

use strict;
use HTTP::Request::Common qw(POST);
use LWP::UserAgent;
use Data::Dumper;

my $ua = LWP::UserAgent->new;
my $req = POST(
    "http://parseerror.com/images/"
    ,"Content_Type" => "form-data"
    ,"Content"      => [
        "image_file" => [ "uploadme.gif" ]
    ]
);

my $res = $ua->request($req);
print $res->as_string;
[download]