A few random thoughts that maybe the purveyor of your library could take note of:

• The new function typically is used for creating new objects. It makes little-to-no sense (to me) for a singleton. Better would be a package method of "get_instance" or something.
• Exposing your singleton object is required in straight-jacket-type languages. Not so much in Perl. Why not just Vendor::Library->some_method(), which would use code like this?

    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 derive
  +d from it?
    }
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  You could put more bondage into that by checking ref $self eq __PACKAGE__ instead of isa, but that's the basic idea. From there, the top of each function would simply look like:

    sub some_method
    {
      my $self = _self_or_create(shift);
      # ...
    }
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  At this point, you're back to where you started, but the user doesn't need to instantiate anything, and then won't be tempted to do silly things like create two of them.
• And for you, maybe not having two creation statements in the same scope? :-)

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

  I have a (probably irrational) fear about explicitly assigning undef. It just smells funny to me. :-)