#!/usr/bin/perl
# printenv.pl
print "Content-type: text/html\n\n";
print "$_\t$ENV{$_}\n" for keys %ENV;
__DATA__

You will see something like this:

COMSPEC="C:\WINNT\system32\cmd.exe"
DOCUMENT_ROOT="C:/Program Files/Apache Group/Apache2/htdocs"
GATEWAY_INTERFACE="CGI/1.1"
HTTP_ACCEPT="image/gif, image/x-xbitmap, image/jpeg, image/pjpeg, application/vnd.ms-powerpoint, application/vnd.ms-excel, application/msword, application/x-shockwave-flash, */*"
HTTP_ACCEPT_ENCODING="gzip, deflate"
HTTP_ACCEPT_LANGUAGE="en-au"
HTTP_CONNECTION="Keep-Alive"
HTTP_HOST="localhost"
HTTP_USER_AGENT="Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.0; .NET CLR 1.0.3705)"
PATH="D:\Perl\bin\;C:\WINNT\system32;C:\WINNT;C:\WINNT\System32\Wbem"
PATHEXT=".COM;.EXE;.BAT;.CMD;.VBS;.VBE;.JS;.JSE;.WSF;.WSH"
QUERY_STRING=""
REMOTE_ADDR="127.0.0.1"
REMOTE_PORT="3264"
REQUEST_METHOD="GET"
REQUEST_URI="/cgi-bin/printenv.pl"
SCRIPT_FILENAME="C:/Program Files/Apache Group/Apache2/cgi-bin/printenv.pl"
SCRIPT_NAME="/cgi-bin/printenv.pl"
SERVER_ADDR="127.0.0.1"
SERVER_ADMIN="admin@hbt.tassie.net.au"
SERVER_NAME="localhost"
SERVER_PORT="80"
SERVER_PROTOCOL="HTTP/1.1"
SERVER_SIGNATURE="<address>Apache/2.0.40 Server at localhost Port 80</address>\n"
SERVER_SOFTWARE="Apache/2.0.40 (Win32)"
SYSTEMROOT="C:\WINNT"
WINDIR="C:\WINNT"