I can recommend loading your file through Win32::OLE and touching up through it, it works like a dream. I used this method to automate proof-reading text layout for a cell-phone for a large number of different languages:

sub SetErrorState {
    my($self) = shift;
    my($Sheet) = $self->{SHEET};
    my($row, $col, $err) = @_;
    my $range = sprintf("%c%d", ord('A') + $col, $row);
    my $cell = $Sheet->Range($range);

    if($err) {
        $cell->ClearComments();
        $cell->AddComment($err);
        $cell->Comment->{Visible} = 0;
        $cell->Interior->{ColorIndex} = 6;
    } else {
        $cell->Interior->{ColorIndex} = xlNone;
        $cell->ClearComments();
    }
}
[download]

You might spot the ColorIndex thing - that was a bitch... Excel version dependant and all...

Cheers,

---Lars