/var/www/devsite/lib
/var/www/testsite/lib
/var/www/prodsite/lib

##</code><code>##

use lib '../lib'; # explicit, hard-coded relative path.

##</code><code>##

use lib "$ENV{DOCUMENT_ROOT}/../lib" =~ m#^(/var/www/[a-z]+site/html/../lib)$#; # path relative to docroot