sub is_circular {
  my $obj = shift; # Object to be traversed
  my $path = shift || [];

  # Check of $obj is in the $path
  foreach my $ancestor (@{$path}) {
    return 1 if $ancestor eq $obj;
  }

  # Check the children recursively, depth first
  # (My imaginary $obj conveniently has 
  # a method for getting an array of its children)
  foreach my $child ($obj->children) {
    return 1 if is_circular($child, [ @{$path}, $obj ] );
  }

  return 0;
}