Don't read UPLOAD to @attach. Pass it directly to MIME::Lite to create MIME message. Here bare bones of untested code:

# create message
use MIME::Lite;
$msg = MIME::Lite->new(
                 From     =>'me@myhost.com',
                 To       =>'you@yourhost.com',
                 Subject  =>'Helloooooo, nurse!',
                 Type     =>'image/gif',
                 Encoding =>'base64',
                 FH       => \*UPLOAD,
                 );
# send it using Net::SMTP
my $smtp = Net::SMTP->new($mailserver);
$smtp->mail ('me@myhost.com');
$smpt->to ('you@yourhost.com');
$smtp->data($msg->as_string);
[download]

See docs for relevant modules.

Update: fixed a couple of bugs and removed Cc: header from MIME message.