Thank you both!

I think this is what I'm going to do:

    use Env;
    ## uid = 0 at this point.

    ## delete root's env
    foreach my $key ( keys %ENV ) {
        delete $ENV{ $key }; 
    }
    ## set standard PATH, apparently some scripts
    ## rely on this
    $ENV{ PATH } = '/bin:/usr/bin:/usr/local/bin:/usr/ucb';

    $> = $target_uid;
    $< = $target_uid;

    my( $shell, $home ) = ( getpwuid( $> ) ) [ 7, 8 ];
    $ENV{ HOME } = $home;
    my $env_str = qx/exec $shell -c env/;

     # now parse
[download]