sub remaining { 
    my ($start, $half, $elapsed) = @_; 
    $remain = $start * (.5**($elapsed/half));
    return $remain;
}

##</code><code>##

print remaining (1000, 10, $_), "\n" for qw (10, 20, 30, 40);
500
250
125
62.5

##</code><code>##

 print remaining (1000, 1, 365), "\n";
1.33061245000255e-107