I agree with this approach, but what I'm seeing (albeit I haven't looked at every single module that mixes in this way), that modules /used/ to be written like this, but aren't anymore, opting to go for simply an XS route and then someone comes along and writes a Pure Perl version. Which is causing me all kinds of headaches.

For example, in MIME::Base64 2.23 (just as an example), you'll see basically what your diagram is sayin':

 
eval { bootstrap MIME::Base64 $VERSION; };
if ($@) {
    # can't bootstrap XS implementation, use perl implementation
    *encode_base64 = \&old_encode_base64;
    *decode_base64 = \&old_decode_base64;

    $OLD_CODE = $@;
    #warn $@ if $^W;
}

# Historically this module has been implemented as pure perl code.
# The XS implementation runs about 20 times faster, but the Perl
# code might be more portable, so it is still here.
[download]

(followed by the Pure Perl version)

In the newest version (as of my writing: 3.07) that's gone, but out has sprouted MIME::Base64::Perl - which is probably the same implementation that *was* in Mime::Base64 to start off with.

Seems like a step backwards.