You can get rid of some whitespace and parens, and also the $a loop variable which you aren't using:

@_=map(1/$_,map($_**2,1..1000000));for(@_){$s+=$_[$i++]}$s*=6;print sq
+rt$s;
[download]

Update: Ooh, and you can save even more by combining some operations:

@_=map(1/$_,map($_**2,1..1000000));for(@_){$s+=$_[$i++]}print sqrt$s*6
[download]