bugiman,
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
</head>
<body>
<form action="http://" name="form">
<input name="series" value="1_1">
</form>
<script>
ss_1_1 = new Array (1,2,3,4,5,6)
sr_1_1 = new Array (1,2,3)
arr_name_1 = 'ss_' + document.form.series.value
arr_name_2 = 'sr_' + document.form.series.value
l = window[arr_name_1].length
alert(l);
l = window[arr_name_2].length;
alert(l);
</script>
</body>
</html>