I was getting the following error while installing rubygems-0.8.11 on my Fedora Core 4 Linux box.
rubygems/custom_require.rb:21:in `require__': no such file
I had installed the latest version of ruby, ruby-1.8.3, using "yum install ruby" with default yum servers.
A google search indicated that I was not the only one facing this problem. Though I couldn't find a fix following the various search results.
Suspecting the problem to be with the particular version of rubygems, I tried an earlier version, rubygems-0.8.10. But no luck. Even this install produced the same error.
I resolved the problem by uninstalling the rpm ("yum remove ruby"), getting the ruby-1.8.3 source tarball and installing it the old fashioned way. ie;
ruby-1.8.3> ./configure
ruby-1.8.3> make
ruby-1.8.3> make test
ruby-1.8.3> sudo make install
Hope this helps others encountering the same/similar problem.