For example:

print "Input radius:\t";
$radius = <STDIN>;
$cirum = $radius * 2 * 3.14159;
print "The circumfrence of the circle with the radius of $radius equal
+s $circum\n";
[download]

By the way, your example, I'm reasonably sure, would work if you just typed in the input. (The program is expecting it, but won't tell you unless it's told to tell you.)