Watch out for file upload fields: if you utf-encode $q->param('upload'), you'll mangle the underlying filehandle. I use an override that's pretty much the same as yours, except that I added the following check:

sub param {
    my $self = shift;
    if (@_ == 1){
        my @val = $self->SUPER::param(@_);
        @val = map { maybe_decode( $_ ) } @val;
    # rest of your param()
}

sub maybe_decode {
    my $p = shift;
    return ( !$p || ( ref $p && fileno($p) ) )
           ? $p
           : encode('utf8', $p);
}
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