Looks to me like the wrong arch-dependent file is being found by DynaLoader. My guess is that either the module upgrade is screwed up, or you have something in your @INC that is shadowing the correct search path.

Do you get the same error if you just try running a simple script like:

use strict;
# BEGIN { $DB::single = 1; }
use MIME::Base64;
print encode_base64('foo'), "\n";
[download]

If so, you can uncomment the BEGIN block and use the Perl debugger to follow what DynaLoader is trying to do. The $DB::single = 1 line will cause the debugger to stop right before the use MIME::Base64 statement gets processed. Then you can just step right in with s, follow to the bootstrap statement in MIME::Base64, and step into that; then you can follow DynaLoader's attempt to find a file to load.