Spreadsheet/ParseExcel.pm lines 1789-1790:
        substr($sWk, 3, 1) &=  pack('c', unpack("c",substr($sWk, 3, 1)) & 0xFC);
        substr($lWk, 0, 1) &=  pack('c', unpack("c",substr($lWk, 0, 1)) & 0xFC);

replace with:
        my $u = unpack("c",substr($sWk, 3, 1)) & 0xFC;
	$u |= 0xFFFFFF00 if ($u & 0x80); # raise neg bits for neg 1-byte value
        substr($sWk, 3, 1) &=  pack('c', $u);
        $u =  unpack("c",substr($lWk, 0, 1)) & 0xFC;
	$u |= 0xFFFFFF00 if ($u & 0x80); # raise neg bits for neg 1-byte value
        substr($lWk, 0, 1) &=  pack('c', $u);