If you find that perlbrew is unable to install perl 5.21.0, and the build log shows the following message:
First let's make sure your kit is complete. Checking... Looks good... *** WHOA THERE!!! *** This is an UNSTABLE DEVELOPMENT release. The version of this perl5 distribution is 21, that is, odd, (as opposed to even) and that signifies a development release. If you want a maintenance release, you want an even-numbered version. Do NOT install this into production use. Data corruption and crashes are possible. It is most seriously suggested that you do not continue any further unless you want to help in developing and debugging Perl. If you still want to build perl, you can answer 'y' now, or pass -Dusedevel to Configure. Do you really want to continue? [n] Okay, bye.
This is a known and resolved issue and has been patched. The simplest way to make things work is to simply say:
$ <span style="text-decoration: underline;">perlbrew self-upgrade</span> $ <span style="text-decoration: underline;">perlbrew install 5.21.2</span> ## works now.