Get cpanspec. Calling it with a CPAN tgz will build the specfile for that package. That's for Fedora, but it will work for other GNU/Linux distros as well, twiddling it a bit. You might want to build cpanspec with itself into a RPM ;-)

Then there is also cpan2rpm. But cpanspec is more up to date.

update: for perl itself, all RPM based GNU/Linux distributions have the source RPM package in their repositories. It includes the spec file. Just get it and install it with rpm -i. Another way to do it: rpm2cpio perl-5.8.8-27.el5.src.rpm | cpio -i perl.spec.To extract the complete source RPM (spec file, tar ball, patches, scripts) just omit the perl.spec.