Carter's compass: I know I'm on the right track when by deleting something, I'm adding functionality

No, that wouldn't help. There has to be a way to discriminate declaration from use if you want to be able to tell a use of a mistyped variable apart from a declaration of a new one. I don't think three characters is a high price to pay for that service - especially considering you can, if the occasion lends itself to it, declare multiple variables with a single my.

Your code has room for clarification and shortening in other areas; I'd write it like this:

sub getDivisionList {
    (my ($agency_name, $nodes_only) = @_) or return;

    my $obj = __PACKAGE__->new($agency_name);
    my @node = $obj->{ts}->get_node_list('division');

    return
        (!@node)    ? () :
        $nodes_only ? ($o, @n) :
            map {($n->value('divname'), $n->value('siebel_id'))} @n;
}
[download]

(The jury's still out on how to indent the dangling last "default" expression, btw. I'm not entirely satisfied with any of the options so far.)

Makeshifts last the longest.

I think I'd layout the return expression like this, but like you, I'm not convinced its necessarially the 'best way'?

  return (!@node)   
       ? () 
       : $nodes_only 
         ? ($o, @n) 
         : map { 
                 ( $n->value('divname'), $n->value('siebel_id')
                 )
           } @n;
[download]

As for the first line of the sub...without actually trying it, I'm not sure quite what the or is checking for. Is that equivalent to

    return unless my($agency_name, $nodes_only) = @_;
[download]

If so, is there an advantage in using the or form?

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Okay you lot, get your wings on the left, halos on the right. It's one size fits all, and "No!", you can't have a different color.
Pick up your cloud down the end and "Yes" if you get allocated a grey one they are a bit damp under foot, but someone has to get them.
Get used to the wings fast cos its an 8 hour day...unless the Govenor calls for a cyclone or hurricane, in which case 16 hour shifts are mandatory.
Just be grateful that you arrived just as the tornado season finished. Them buggers are real work.