Thanks to all who bothered to answer my first posting. Unfortunately, my problem persists. Here are a few more details.
There is a static front page index.html where the user is asked to select one of two possible languages. There are two links on this first page: /cgi-bin/index.cgi/en, /cgi-bin/index.cgi/fr.
Below is the code for index.cgi. The warn message there shows up twice in my error log, the first time with language set to the correct value, and the second time with language=undefined...
I'm using CGI::FastTemplate to generate the HTML (it's a use-clause in mysession.pm).
Note the init_session($language) call. It checks for a SESSION_ID cookie, and then ties a sesion hash to an Apache::Session::File.

use mysession;

my $language; 

warn ("\n In index.cgi language=$ENV{'PATH_INFO'} \n");

if (defined ($ENV{'PATH_INFO'}) ) {
     $language = uc(substr($ENV{'PATH_INFO'}, 1)); 
 }
else {
     $language='en'; 
 }
 
init_session($language);
assign(LANGUAGE=>lc($language));
define(main=>'index.html');
parse(MAIN=>'main');
nice_exit();
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