sub x{ 
    my( $i, $r, $d, $o ) = @_; 
    return $i / $d**$o; 
}

print x( 100, 0, $_, 0.5 ) for map 2**$_, 4..13;;
25
17.6776695296637
12.5
8.83883476483184
6.25
4.41941738241592
3.125
2.20970869120796
1.5625
1.10485434560398

print x( 100, 1000, $_, 0.5 ) for map 2**$_, 4..13;;
25
17.6776695296637
12.5
8.83883476483184
6.25
4.41941738241592
3.125
2.20970869120796
1.5625
1.10485434560398