my $final =  Spreadsheet::WriteExcel->new('filename.xls');

for my $xls ( 'first.xls', 'second.xls' ){
    my $book = ...->read('second.xls');
    my $worksheet = $book->Worksheet(0);
    my $sheetf = $final->add_worksheet( $worksheet->{Name} );

    my ( $row_min, $row_max ) = $worksheet->row_range();
    my ( $col_min, $col_max ) = $worksheet->col_range();

    for my $row ( $row_min .. $row_max ) {
        for my $col ( $col_min .. $col_max ) {

            my $cell = $worksheet->get_cell( $row, $col );
            next unless $cell;

            $sheetf->write( $row , $col, $cell->unformatted  );
        }
    }
}