var gljson; //GLOBAL VARIABLE

var url = '/my/path/here';
var ajobj = new Ajax.Request(
   url,
   {
       method: 'get',
       onComplete: setMakeVars
   });

function setMakeVars (transport) {
  var json = eval(transport.responseText);
  gljson = json;
  alert(gljson) // it *is* defined with the correct value
}

alert(gljson); // now it is undefined

////////////////////////////////////////////////////

var url = '/util/make_select_response';

var gljson;
var mk_keys;
var mk_vals;

var ajobj = new Ajax.Request(
    url,
    {
        method: 'get',
        onComplete: setMakeVars.bind(this)
    });

function setMakeVars (transport) {
   var json = eval(transport.responseText);
   jslog.warning("in setMakeVars: " + json);
   gljson = json;
   jslog.warning("in setMakeVars gljson: " + gljson);
}

jslog.warning("glson: " + gljson);
mk_keys = $H(gljson).keys();
mk_vals = $H(gljson).values();
jslog.warning("mk_keys: " + mk_keys);


new Ajax.InPlaceSelect('someId', 'someURL', mk_keys, mk_vals,
    { paramName: 'asset_type', parameters: "moreinfo=extra info" } );