bool next(Type& arg) {
   if (...) {
      return false;
   } else {
      arg = ...;
      return true;
   }
}

while (i->next(arg)) {
   ...
}

##</code><code>##

sub next {
   my  $self            =  $_[0];
   our $arg; local *arg = \$_[1];
   if (...) {
      return 0;
   } else {
      $arg = ...;
      return 1;
   }
}

while ($i->next($arg)) {
   ...
}

##</code><code>##

sub next {
   my ($self) = @_;
   if (...) {
      return;
   } else {
      return ...;
   }
}

while (my ($arg) = $i->next()) {
   ...
}

##</code><code>##

while (my $arg = $i->next()) {
   ...
}