Why not just subclass CGI?

If you don't want to do that, then I recommend the following:

# Must appear before "use CGI".
BEGIN {
   require CGI;
   package CGI;
   no warnings 'redefine';
   my $real_param = \&CGI::param;
   sub param {
      if (wantarray) {
         return map { Encode::decode_utf8($_) } $real_param->(@_);
      } else {
         return Encode::decode_utf8($real_param->(@_));
      }
   }
}
[download]

My version fixes two bugs.

• My version works when called as a function.
• My version works correctly when called in list context.

Updated to work in list context.