put this code above <do action...> <perl> if(%_put){

@needed_fields=("Fieldname 1","Fieldname 2"); # you can add more fields...
foreach(@needed_fields){
    if(!$_put{$_}){
      push @missed,$_;
    }
  }
  if (@missed){
    out "Error:<br>".join("<br>",@missed)."<br>This fields are required";
    $cmd=$_get{cmd}="";
    undef(%_put);
  }

}