while (1) {
    $tmpl =~ /\G(...)/gcs  && do {
       if ($1 eq "w") {
           croak "w is bad";
       } else {
           $result .= process_template($1);
       }
    }
 || $tmpl =~ /\G(...)/gcs  && do {
       ...do.something.else...
    }
 || last;
}