{
  package PKG;
  sub safe_sub {
     my $self = shift if @_ == 3
                        and UNIVERSAL::isa($_[0], __PACKAGE__)
                        or $_[0] eq __PACKAGE__;
     my($hash, $string) = @_;
     print $hash->{foo}, ':', $string, $/;
  }
  sub new { bless {}, shift }
}

my $obj = PKG->new();

print '$obj->safe_sub - ';
$obj->safe_sub({'foo' => 'bar'}, 'cool');
print 'PKG->safe_sub  - ';
PKG->safe_sub({'foo' => 'bar'}, 'cool');
print 'PKG::safe_sub  - ';
PKG::safe_sub({'foo' => 'bar'}, 'cool');

package PKG;

print 'safe_sub       - ';
safe_sub({'foo' => 'bar'}, 'cool');

__output__

$obj->safe_sub - bar:cool
PKG->safe_sub  - bar:cool
PKG::safe_sub  - bar:cool
safe_sub       - bar:cool