http://www.perlmonks.org?node_id=430235
Description: Validate based on whether $ENV{REMOTE_ADDR} is present in a list of IP's or in the IP ranges listed in CIDR notation. Real-world application was within a subfunction, used to restrict signup for an email account.
#!/usr/bin/perl

use CGI; 
use Net::CIDR;

open ACL, "< /usr/local/atmail/ACL.txt";

my (@ACL, @CIDRs, $entry, $entry1, $offset, $offset1, $ip, $found);

# Create an array of the entries from the list file
while(<ACL>){
    push(@ACL, $_);
}

# Push the CIDR's into an array and undef them (avoiding splice() prob
+lems with the foreach and offset) in @ACL so they can be easily remov
+ed later...
foreach $entry (@ACL){
        if ($entry =~ /(\d{1,3}\.){3}\d+\//){
        push(@CIDRs, $entry);
        undef $ACL[$offset];
        }
        $offset++;
}

# Anything left in the list that's not an IP gets whacked
foreach $entry1 (@ACL){
        if (!(Net::CIDR::cidrvalidate($entry1))){
                undef $ACL[$offset1];
        }
        $offset1++;
}

# Iterate ACL, finishing up if the IP in the HTTP request can be found
+ in any of the CIDRs or if it matches the current IP in the list.
foreach $ip (@ACL){
    chomp $ip;
    if ($ENV{REMOTE_ADDR} eq "$ip" || Net::CIDR::cidrlookup($ENV{REMOT
+E_ADDR}, @CIDRs)){
        $found = 1;
        last;
    }
}

# If your IP is allowed,  do nothing.  If not,  display error message.
if ($found){
}
else{
print "Content-Type: text/html\n\n";
print "ERROR YOU CANT SIGNUP";
exit;
}