This script is cloaking broken down to it's simplest level. All you need to do is know your IP address (you could find it by going to the link in my profile) and put your IP in the script where it says "Put_your_IP_here". Then run the script with a dummy IP (don't leave it blank) and watch the change. It all starts with this simple logic.
#!/usr/bin/perl
print "Content-Type: text/html\n\n";
my $RA = "$ENV{'REMOTE_ADDR'}";
if ($RA =~/Put_your_IP_here/) {
print "I think you are a spider";
}
else {
print "I think you are a human";
}