Thanks for the quick replies!
I poked around CGI.pm and discovered that param() is already calling Encode::decode_utf8, so I don't need to! But decode is only called if the charset() is set to utf-8.
So I also tried the following, which works sometimes, and sometimes not. I'm not sure why the charset gets unset sometimes. In the end I used ikegami's solution. Thanks ikegami!

BEGIN{
  no strict 'refs';
  my $real_new = \&CGI::new;
  *{"CGI::new"} = sub {
      my $cgi = &$real_new(@_);
      $cgi->charset('utf-8');
      return $cgi;
  };
}
[download]

However, I am confused about one thing in the code suggested.

I always thought that if you take a reference, and then change the underlying typeglob, then the referent must now refer to the new typeglob. The above $real_new refers to the CODE slot in the CGI::new typeglob, and then I put a new sub into the CODE typeglob, so why does $real_new still refer to the contents of the old CODE slot?

And I'm still intersted in seeing out how to do this by using a typeglob instead of a reference to a sub.

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