Well, I got it working. I ended up making my own serialize function in J/S.

Just in case anyone needs it, here's what I did.

On the server:

my @selected = map { uri_unescape($_) }split(/,/, $cgi->param($attr));

On the client:

document.getElementById("Finish Codes").value = serialize(document.get
+ElementById("Finish Codes rightbox").options);
document.getElementById("Process Codes").value = serialize(document.ge
+tElementById("Process Codes rightbox").options);
.
.
.
function serialize(list) {
   var encodedlist = new Array(list.length);
   for (var i = 0; i != list.length; i++)
   {
      encodeURIComponent(list[i].value);  
      encodedlist.push(list[i].value);     
    }
   return encodedlist.join(",");
}
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