my $filename = "largefile.avi";
open(FH,"<$filename");
my $upload_cb = sub {
        if ( sysread FH, my $buffer, 4096 ){
                return $buffer; 
        } else {
                close FH;
                return ''; 
        }
}
# later in the Code, when uploading the File, calling it like this
$vid_fields{field_uploadfile}             =  [ $upload_cb ] ;
$vid_fields{field_private_share_entities} = "";
$resp = upload_step_two($action_url, \%vid_fields);