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";