Well, DUH! use the actual path, then! I used the path you gave in your question and I figured you'd be smart enough to adapt it to the right value yourself.

Just to make this absolutely clear, if your module is located in /opt/mrn/Foo/Bar/Baz.pm
and the package line says:
package Bar::Baz:, then you need to say>
use lib "/opt/mrn/Foo";