+-------------------+
|XS implementation  |
|Either install this| ---requires--\
+-------------------+              |
                                   V
 +-------------------------------------+
 |pure-Perl module distribution        |
 |Or install this                      |
 |Other tools require this distribution|
 |  +---------------------------+      |
 |  |pure-Perl interface package|      |
 |  |Only package used in code  |      |
 |  +---------------------------+      |
 | +--------------------------------+  |
 | |pure-Perl implementation package|  |
 | +--------------------------------+  |
 +-------------------------------------+

(or)

+-------------------+
|XS implementation  |
|Either install this| ---requires--\
+-------------------+              |
                                   V
                    +--------------------------+
                    |pure-Perl interface module|
                    |Or install this           |
    /---requires--- |Only package used in code |
    |               |Other tools require this  |
    V               +--------------------------+
+-------------------------------+
|pure-Perl implementation module|
+-------------------------------+