How about this (72 strokes - about par for the course ;-) for($i;$i<100000;$i+=2){$_.="+4/(($i*2)+1)-4/((($i+1)*2)+1)"};print eval

I'm assuming the algorithm can't be changed.
I just got rid of the neat closure and built it all in the loop. It's got the same five digits of accuracy. The same 72 strokes can have up to a whopping 6 digits of accuracy if you change the 100_000 to 999_999!

jsprat