#!/usr/bin/perl

use Fcntl qw(:DEFAULT :flock);
use Digest::MD5  qw( md5_base64 );
use strict;
use vars qw( $referer $counter_file $ofh $num );
$referer = $ENV{DOCUMENT_URI};
$counter_file = "/home/httpd/(your_site)/var/counter/". md5_base64($referer);

sysopen(FH, $counter_file, O_RDWR | O_CREAT) or die "can't open numfile: $!";
$ofh = select(FH);
$| = 1;
select ($ofh);
flock(FH, LOCK_EX) or die "can't write-lock numfile: $!";
$num = <FH> || 0;
seek(FH, 0, 0) or die "can't rewind numfile : $!";
print FH $num+1, "\n" or die "can't write numfile: $!";
truncate(FH, tell(FH)) or die "can't truncate numfile: $!";
close(FH) or die "can't close numfile: $!";

print "Content-type: text/html\r\n\r\n";
print "Counter for $referer is $num\n";