I think you'll need to change

my $mask = "1" x ( 32 - $cidr ) . "0" x $cidr;
[download]

to

my $mask = "1" x $cidr . "0" x ( 32 - $cidr );
[download]

since /8 means a mask of 255.0.0.0 and not 255.255.255.0 which is /24.
But thanks a lot for this example - I was wondering how do that without a module (and could not figure how bitwise shifting should help).