Eureka! I have since found the problem is due to the fact that  @hsbcdataread = $sheets[1]->next_row is returning an array of less than the expected size of 6 columns. For some reason, sometimes it returns six columns where one is empty, and other times it just returns 5 columns instead, meaning that the foreach loop is not always entered into for the sixth column in my Excel spreadsheet. A quick and messy if statement has solved this problem. As is often the case, the problem was not what it initially appeared to be. Many thanks to those who contributed. New Code:

while ($sheets[1]->has_data)
{
  @hsbcdata = ();
  @hsbcdataread = $sheets[1]->next_row; 
  $size = @hsbcdataread;
   
  if ($size < 6)
  {
    $hsbcdataread[5] = "X";
  }

  foreach $_ (@hsbcdataread)
  {
    s/^\s*//;
    s/\s*\r*$//;
    chomp ($_);

    if (/\S+/)
    {
      push (@hsbcdata, $_);
    }

    else
    {
      push (@hsbcdata, "X");
    }
  }
.
.
}
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