Applying it to two sites 

• example.org

PerlAddVar MasonCompRoot "main   => /usr/share/example.org/mason"
PerlAddVar MasonCompRoot "common => /usr/share/site-common/mason"

• example.com

PerlAddVar MasonCompRoot "main   => /usr/share/example.com/mason"
PerlAddVar MasonCompRoot "common => /usr/share/site-common/mason"

• calling show_entry.mhtml (same as usual):

<& /blog/comp/show_entry.mhtml, entry => $entry &>