To build a CPAN module as an RPM

Use cpanspec.

$ cpanspec -b -v String::Random
Updating /home/steve/.cpan/sources/modules/02packages.details.txt.gz...
Fetching /home/steve/.cpan/sources/modules/02packages.details.txt.gz from http://www.cpan.org//modules/02packages.details.txt.gz...
/home/steve/.cpan/sources/modules/02packages.details.txt.gz is up to date.
Fetching String-Random-0.20.tar.gz from http://www.cpan.org//authors/id/S/ST/STEVE/String-Random-0.20.tar.gz...
Building rpms from perl-String-Random.spec
[...]
Wrote: /home/steve/tmp/cpanspec/perl-String-Random-0.20-1.src.rpm
Wrote: /home/steve/tmp/cpanspec/noarch/perl-String-Random-0.20-1.noarch.rpm
Executing(%clean): /bin/sh -e /home/steve/src/redhat/tmp/rpm-tmp.19562
+ umask 022
+ cd /home/steve/tmp/cpanspec
+ cd String-Random-0.20
+ rm -rf /home/steve/src/redhat/tmp/perl-String-Random-0.20-1-root-steve
+ exit 0
$ ls -F
String-Random-0.20/        perl-String-Random-0.20-1.src.rpm
String-Random-0.20.tar.gz  perl-String-Random.spec
noarch/

(perl-String-Random-0.20-1.noarch.rpm is in the noarch directory.)

FWIW, the generated spec file may need a bit of work to actually build, but cpanspec can usually do the vast majority of the work for you.