#!perl -w<P>

 use CGI qw/:all -nph/;
 $POST_MAX=100;
 $DISABLE_UPLOAD=1;
 $q=new CGI;<P>

#et cetera..<P>

$pattern=qr/(\w-]+\.){2,5}[a-zA-Z]{2,4}\b)/;
$good_param_qs=&taint_param('qs',$pattern );<P>

#do staff with almost-tainted param qs...............<P>

sub  taint_param()
 {
  $parametro=shift;
  $matching=shift;
  if ($q->param($parametro)=~/($matching)/){return $1}
  else{print"Invalid parameter!\n\n";return undef}
 }