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# this marks the current vertex
$explored->{$node->{_id}}++;

...
   # not of the vertex!!
   $do_search->($link->{_to}) unless ($explored->{$link->{_id}});
}

$explored->{$node->{_id}}++;
foreach my $link (@{$node->{_outlinks}}) {
   my $new_node = $link->{_to};
   $do_search->($new_node) unless ($explored->{$new_node->{_id}});
}