...the error message itself is pretty simple

Indeed it is. But I'm not sure how this observation helps the OP. Lines 1848-1850 of SpreadSheet::ParseExcel are:

for(my $i = ($iDiff/2); $i>=1;$i--) {
  substr($sWk, $i+1, 0) =  "\x00";
}
[download]

Looks like a bug in the module to me, unless svar is doing something very strange when using it.