package Foo;
sub aa {
  my ( $self, $args ) = @_;
  my $val = $self->_aa ( $args->{'bar'} );
}
sub _aa {
  my $self, $quux ) = @_;
}
sub bb {
  my ( $self, $args ) = @_;
  my $val = $self->_bb ( $args->{'foo'} );
}
sub _bb {
  my $self, $fazoo ) = @_;
}

##</code><code>##

package Foo::Bar;
sub _aa { my $self, $quux ) = @_; }
..

##</code><code>##

package Foo;
sub aa {
  my ( $self, $args ) = @_;
  my $val = Foo::Bar::_aa ( $self, $args->{'bar'} );
}
..