Looks like yet another example, similar to this recent thread, of problems experienced when you depend on the OS version of Perl.

To avoid further pain as AIX Perl evolves, I suggest you don't use it. Build your own completely separate version of Perl, including CGI, and use that instead -- see Re^7: Meaning of XS object version (Package Manager Security References - example building Perl securely from source) and this reply for examples of building your own local perl securely.

As for why, see Re^4: Meaning of XS object version and other replies to Meaning of XS object version.

Note that CGI.pm is no longer in the Perl core (the last Perl core that included it was perl v5.20).

Updated: Added Re^7: Meaning of XS object version (Package Manager Security References - example building Perl securely from source) - an example of building your own local perl securely from scratch.