You can install part of the IBM toolbox, containing Perl 5.8 packages. Download them from
ftp://ftp.software.ibm.com/aix/freeSoftware/aixtoolbox/RPMS/ppc/perl/
Note that some AIX versions are previously bundled with Perl v5.6: This version is able to run Pandora FMS agents, so you won’t need to install Perl 5.8.