Here's how I do it:

This JavaScript is written to the page:

<script type="text/javascript" language="JavaScript">
<!--
document.write("<img src=\"ref.cgi?refer=");
document.write(escape(document.referrer));
document.write("\" width=1 height=1>");
// -->
</script>
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This CGI runs on the server (Pip.png is a 1 pixel graphic):

#!/usr/bin/perl

use CGI qw(:standard);
my $pip  = '/path/to/pip.png';
my $logf = '/path/to/referrer.log';

my $ref = param("refer");

open(LOG, ">>$logf");
print LOG $ref;
close(LOG);

open(PIP, "<$pip");
undef $/;
my $pixel = <PIP>;
close(PIP);
$/ = "\n";

print header( -type=>'image/png' );
binmode(STDOUT);
print STDOUT $pixel;
exit;
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