$ ./pm_1216483_loop_qanda.pl
Type radius (>=0) or just hit Return to exit.
Radius: 123.456
Circumference: 775.696925283163
Radius: q
Invalid radius [q]. Try again.
Radius: 0
Circumference: 0
Radius: +0
Circumference: 0
Radius: -0
Circumference: 0
Radius: 0.00000001
Circumference: 6.28318530717959e-08
Radius: +0.00000001
Circumference: 6.28318530717959e-08
Radius: -0.00000001
Radius [-0.00000001] is less than zero. Try again.
Radius: .00000001
Circumference: 6.28318530717959e-08
Radius: +.00000001
Circumference: 6.28318530717959e-08
Radius: -.00000001
Radius [-.00000001] is less than zero. Try again.
Radius: 1
Circumference: 6.28318530717959
Radius: -1
Radius [-1] is less than zero. Try again.
Radius: 1.
Invalid radius [1.]. Try again.
Radius: .1
Circumference: 0.628318530717959
Radius: load of rubbish
Invalid radius [load of rubbish]. Try again.
Radius: 
$