I like this module, and Ensure is an appropriate name IMO, but Loop::Invariant describes more clearly what the programmer intends to happen. Contract is a close third, while I find Assert unexpected in the sense that the program dosen't die when an assertion fails.

But I have some problems with running your module :

1. It dosen't run under Perl 5.5.3 (ActiveState Perl build 517)
2. Under Perl 5.6.0 (IndigoPerl), I get Can't use string ("") as a subroutine ref while "strict refs" in use at Ensure.pm line 56. as an error in the Ensure::Scalar::FETCH sub. I think this comes from a conflict in execution order as follows :
   1. Ensure::using is called to tie the scalar, and sets up the Ensure::Scalar::FETCH handler.
   2. Ensure::ensure is called, and itself calls tied($$_), which in turn results in Ensure::Scalar::FETCH being called.
   3. Ensure::Scalar::FETCH is called and runs into the undef, as the handlers have not yet been set up (from within Ensure::ensure).

The quick fix to me was a small change to the Ensure::Scalar module as follows :

sub TIESCALAR {
  my ($class, $val) = @_;
  my $ref = [ $val, "setup", "setup" ];
  bless $ref, $class;
  return $ref;
}

sub FETCH {
  my $self = shift;
  my $val = $self->[0];
  if (ref($self->[1]) eq 'CODE') {
    die "[Ensure]\n" unless $self->[1]->();
  };
  return $val;
}
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This fix would have to be repeated for the other modules (or maybe isn't necessary at all with Perl 5.6.1), and I'm not sure if my fix is really a fix and dosen't introduce more errors, as this is the first time I've dabbled with tie :-)