The default version of Perl on Katana is 5.10.1 which is provided by CentOS 6 and can be found in /usr/bin.
This version of Perl is used throughout the system by CentOS. For example, system tools such as logwatch require /usr/bin/perl. In order to avoid the issue of a poorly written Perl module breaking important system tools there are multiple versions of Perl which have been installed and are available via the module command.
Currently Perl 5.18.0 and Perl 5.20.1 include a number of additional Perl modules such as BioPerl are available via the module command (by loading either one of the Perl modules or one of the BioPerl modules). If you wish to use one of these versions of Perl for your own code (or for code that you have obtained) you should make sure that the first line is
otherwise it will use system Perl rather than the module version.