You can create file .htaccess and define there path to your library:

PerlSetEnv PERL5LIB /path/to/your/library
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But notice that this should be allowed do in http.conf:

<Directory "/var/www/html">
   Options Indexes FollowSymLink
   AllowOverride All # !!!!!!
   Order allow,deny
   Allow from all
</Directory>
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In this case you can change this path without restart Apache.

Also, you can create some start up script, where you can define path to your libraries:

use lib qw('/path/to/your/library');
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But notice that if you change this path you should restart Apache!

I think it will be useful for you to see .htaccess files howto and CGI to mod_perl Porting. mod_perl Coding guidelines.

Hope it will help you.