I wrote the following in JavaScript a few years ago. The logic should be the same in Perl, but I'll leave porting it as an exercise for the reader.

  var query_string = {};
  var query = window.location.search.substring(1);
  var vars = query.split("&");
  for (var i=0;i<vars.length;i++) {
    var pair = vars[i].split("=");
        // If first entry with this name
    if (typeof query_string[pair[0]] === "undefined") {
      query_string[pair[0]] = pair[1];
        // If second entry with this name
    } else if (typeof query_string[pair[0]] === "string") {
      var arr = [ query_string[pair[0]], pair[1] ];
      query_string[pair[0]] = arr;
        // If third or later entry with this name
    } else {
      query_string[pair[0]].push(pair[1]);
    }
  }
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