You are constrained to use Javascript. All Perl can do is give you a new full size window.

# you can force a new full size window like this:

<form method="POST" action="$cgi" target="_new">
....
</form>

# or using javascript you can do stuff like:

<script language="JavaScript">
<!-- Begin
var x = 25;
var y = 25;
function popUp(URL) {
    x += 25;
    y += 25;
    window.open(URL,"","toolbar=0,scrollbars=1,location=1,statusbar=0,
+menubar=0,resizable=1,width=750,height=600,left="+x+",top="+y);
}
// End -->
</script>

<a href="javascript:popUp('http://domain.com/cgi-bin/myscript.pl?foo=b
+ar')">pop little window</a>
[download]

This particular function pops a new window at (50,50) for the first call (75,75) for the next, etc so they cascade. Hard code x,y as desired.