package Foo;
require Foo::Bar; # load additional methods
sub aa {
  my ( $self, $args ) = @_;
  my $val = $self->_aa ( $args->{'bar'} );
}
sub bb {
  my ( $self, $args ) = @_;
  my $val = $self->_bb ( $args->{'foo'} );
}
1;

##</code><code>##

package Foo;  # not Foo::Bar!!!
sub _aa {
  my ( $self, $quux ) = @_;
  ...;
}
sub _bb {
  my ( $self, $fazoo ) = @_;
  ...;
}
1;

##</code><code>##

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