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package Foo;
require Foo::Bar; # load additional methods
...
  my $val = $self->_bb ( $args->{'foo'} );
}
1;

package Foo;  # not Foo::Bar!!!
sub _aa {
...
  ...;
}
1;

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