Every poster was correct pointing out that $session was not defined in my calling program. Duh!

So, I tried this first:

my $logged_in = write_session( $user );

my $session = CGI::Session->load();
print Dumper($logged_in, $session->param('user'));
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which I thought would be the proper way to do it, but ended up following your advice with:

my $session = new CGI::Session();
my $logged_in = write_session( $session, $user );

print Dumper($logged_in, $session->param('user'));
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which did the trick. Thanks.