...which was supposed to leave perl untouched.

Apparently, that's not true, as

  MIME::Base64 object version 2.12 does not match bootstrap parameter 
+3.07 at ...
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means that the associated shared object file (XS part, presumably /usr/opt/perl5/lib/5.8.0/aix-thread-multi/auto/MIME/Base64/Base64.so) of the module MIME::Base64 does no longer match the version requested in the MIME/Base64.pm file... (3.07 requested, 2.12 found)

You might want to check whether you have any unwanted search paths in @INC:

$ perl -V
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