If you change the shebang line to #!/usr/bin/perl -w (notice the lowercase w), this error does not occur. I'm not sure what the difference is between lowercase and uppercase w in the shebang line but I'm looking that up now.
Update
-w prints warnings...
-W "Unconditionally and permanantly enables all warnings throught the program, even if warnings were disabled locally using no warnings or $^W = 0. This includes all files loaded via use, require, or  do." from the Camel Book

I haven't checked but it could be possible that the author turned off warnings intentionally.

I use

use MIME::Parser;
use MIME::Entity;
use MIME::WordDecoder;
use MIME::Body;
use MIME::Base64;
use MIME::QuotedPrint;
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quite a bit in a an email program I wrote and have no problems or warnings but then again I use -w instead of -W