Further clarification: Just to test this out I have now changed the write_string() code to:

if($y == 1 && $x == 1)
  {
    # In my test case only cell (1,1) causes a 
    # problem
    my $t = "...Long string with the value I expect...";

    if($val eq $t)
      {
        print "Equal\n";
      }
    else
      {
        print "NOT EQUAL\n";
      }
    if($fail_to_open)
      {
        $r = $sheet->write_string($y,$x,
                           $val,$formats{$format_name});
      }
    else
      {
        $r = $sheet->write_string($y,$x,
                           $t,$formats{$format_name});
      }
  }
else
  {
    $r = $sheet->write_string($y,$x,
                            $val,$formats{$format_name});
  }
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The program asserts that $val and $t are eq. The spreadsheet fails if I use $val and works if I use $t.