my $_global;
  sub _self_or_create
  {
    my $self = shift;
    return $self if (ref $self and $self->isa(__PACKAGE__))
    $_global ||= __PACKAGE__->_new();
    return $_global; # assume $self eq __PACKAGE__ or something derived from it?
  }

##</code><code>##

  sub some_method
  {
    my $self = _self_or_create(shift);
    # ...
  }

##</code><code>##

{
  my $obj = Vendor::Library->new(...);
  # code here
}
{
  my $obj = Vendor::Library->new(...);
  # different code here (I presume)
}