sub _private_sub {
   die "_private_sub is private!" unless caller eq __PACKAGE__;
   # private stuff #
}

##</code><code>##

sub PROTECTED {
  my $self = shift;
  
  my @caller = caller(1);
  $caller[3] =~ m{(.*)::(.*)$};
  my ($pack, $sub) = ($1,$2);
  
  local $Carp::CarpLevel += 1;
  confess "method '$sub' is protected by '$pack'" 
   unless ($caller[0]->can($sub));
  
  return $self;
}

##</code><code>##

use base 'BaseClass';
# some stuff #

sub _protected_sub {
   my $self = shift->PROTECTED;
   print "This is a protected sub!";
}

##</code><code>##

method 'test2' is protected by 'LocalTest' at LocalTest.pm line 12
        LocalTest::test2('LocalTest2=HASH(0x18633a4)') called at test.pl line 15
        main::test() called at test.pl line 20