The 'classic' Perl private method hack (IIRC) is an anonymous subroutine reference (update: stored in a lexical my variable) in a module (or indeed, within the limitation of any scope).

c:\@Work\Perl>perl -wMstrict -le
"my $ps = sub { my $class = shift;  print qq{obj/class: $class (@_)} }
+;
 ;;
 Foo->$ps;
 'Quux'->$ps(1, 2, 3, 'foo', 'bar');
 ;;
 my $o = bless [];
 $o->$ps(9, 8, 7);
"
obj/class: Foo ()
obj/class: Quux (1 2 3 foo bar)
obj/class: main=ARRAY(0x35c06c) (9 8 7)
[download]

See The Arrow Operator in perlop.

Update 1: Of course, that's just your basic, OOTB Perl. If you avail yourself of one of the many, fine OO support/extension modules/bundles, you may find explicit private method support. Can't think of an example offhand...

Update 2: Reading over this thread again, I notice now that LanX has already covered this hack above. Oh, well...