$ENV{REMOTE_ADDR}
[download]

For that matter, you could use remote_host() from CGI.pm.

Also, try this:

#!/usr/bin/perl -w
use strict;
$|++;

use CGI;

my $cgi = CGI->new;

print $cgi->header,
      $cgi->start_html,
      $cgi->pre(
                ( map { "$_\t$ENV{$_}\n" } keys %ENV )
               ), 
      $cgi->end_html;
[download]

That was untested, but I'm pretty sure it's right :-)