Has already been unwrapped into directory /home/uce/.cpan/build/DB_File-1.838-yBmp3Q

Not sure ... I'm not at all that familiar with these cpan* shells.
However, I think it means that it wouldn't unpack PMQS/DB_File-1.838.tar.gz into /home/uce/.cpan/build/DB_File-1.838-yBmp3Q because that directory already exists.
That being so, removing that DB_File-1.838-yBmp3Q directory should work. (Or perhaps opening a new instance of the CPAN shell and re-running the same command will unpack into a different location and therefore succeed.)

Cheers,
Rob