As Davido has mentioned, subclassing would be a better approach. On the other hand, if you don't want to do that, then this should work:-

{
    no strict 'refs';
    no warnings 'redefine';

    my $code = \&CGI::param;
    *{"CGI::param"} = sub {
        my $ret = &$code(@_);
        return Encode::decode_utf8($ret);
    };
}
[download]

There are also various modules on the CPAN to do this for you - have a look at Hook::LexWrap and Sub::WrapPackages for starters.