sub input_validation
{
  my $your_name = $query->param("user_name");
  my $your_sex = $query->param("user_sex");

  my $error_message = ""; # holds error message

  #Here do whatever validation you want to do
  $error_message .= "Please enter your name<br>" if ( !$user_name );
  $error_message .= "Please specify your sex<br>" if ( !$user_sex );

  if ( $error_message )
  {
    show_login_form ( $error_message, $user_name, $user_sex );
    return 0;
  }
  else
  {
    # Form OK
    return 1;
  }
}

sub show_login_form
{
  my ($error_message,$user_name,$user_sex) = @_;
 
  
  #Add user login or signup HTML content here..
  <form>
   .....
   ......

  <p>$error_message</p>

  </form>
}