package Foo;
sub new { ... };
sub xyzzy { ... };
1;

##</code><code>##

require Foo;
my $foo = new Foo ( );
print $foo->xyzzy ( );

##</code><code>##

# The new Foo.
package Foo;
require System::Foo;
@ISA = qw ( System::Foo );
...
1;

##</code><code>##

package System::Foo;
...