system("/usr/sbin/useradd $cli");

##</code><code>##

  "... ; /bin/rm -rf *"

##</code><code>##

  my $username = ... untaint $formdata{'username'};
  ... etc. for other form variables

  my @cli = ("-s", $shell,
             "-d", $home,
             "-G", $group
             $usrname);
  ...
  system("/usr/sbin/useradd", @cli);