Why it is so hard to install a CPAN module?

Because you are using the wrong compiler!

It seems that your Perl was not compiled with gcc but with SUNOracle compiler. Trying to compile modules with a different compiler does not work because it uses different command line switches and options (... it can be done, but requires fiddling with Config).

Download the Oracle compiler from its web, it is (or used to be) free, gratis.

Alternatively, build a new Perl with gcc from source or download it as a Solaris package from SUN Freeware and use it instead of the old and buggy perl 5.8.4 that comes preinstalled on your system.