And so others can get the benefit of your wisdom w/o the OOPerl book (but buy it nonetheless, its a great book), pg 34 gives the example:

sub listdir
{ 
  # do file listing and the:

  return @missing_files if wantarray();
  return $listed_count if defined(wantarray());
  carp "subroutine &listdir was called in a void context";
}     # sub listdir
[download]

The really cool thing here: wantarray returns true if:
@missing = listdir(@files);
(called in a list context); returns "" (i.e. false but defined) if:
$list_count = listdir(@files);
(called in a scalar context) and returns undef if:
listdir(@files);
(called in a void context - get the carp).

a