next does not a loop make.

The error is pretty clear, I think. You don't have a loop, so you can't next. You need to create a loop, like below. Note that you didn't have any checking for odd conditions, such as $key and $self->{FIRSTKEY} never matching. Thus, you could have run off the end of @KEYS (which I assume you define elsewhere), creating an infinite loop and lots of warnings about undefined values.

sub NEXTKEY {
  my $self    = shift;
  #my $lastkey = shift;  # Do you need this?  If not, don't bother shi
+fting it

  until($self->{FIRSTKEY} eq $KEYS[$self->{INDEX}]) {
    ++$self->{INDEX};
    last if $self->{INDEX} >= $#KEYS;  # Quit if we run out of @KEYS
  }

  return($KEYS[$self->{INDEX}]);
}
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PS, you should probably wrap any code you post here in <CODE> </CODE> tags, so that any special characters, like <, >, [, and ] are properly escaped.

bbfu
Black flowers blossum
Fearless on my breath

next</next> in an iteration statement (<code>foreach, for or while) means that all further statements in the body of the loop are skipped and that the next iteration is started.

In the code snippet above there's no loop statement that I can see, so next doesn't make any sense semantically, and that's exactly what the interpreter is telling you.

Hope this helps, -gjb-

sub NEXTKEY
{
     my ($self, $lastkey) = @_;  # $lastkey isn't used?
     while (my $key = $KEYS[$self->{INDEX}++])
     {
          if ($key ne $self->{FIRSTKEY})
          {
               return $key;
          }
     }
}
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Like everybody else said, there are better ways to do this. I'd just like to note that if you, for some reason, don't want to use any of the normal looping constructs, you can use a do{} with no while() clause:

sub NEXTKEY {
  my $self=shift;
  my $lastkey=shift;
  do {
    $key=$KEYS[$self->{INDEX}++];
    next if ($key eq $self->{FIRSTKEY});
    return($key);
  }
}
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