Your derivation of $iFmt does not match the example provided at http://search.cpan.org/~jmcnamara/Spreadsheet-ParseExcel/lib/Spreadsheet/ParseExcel/SaveParser.pm (q.v.), which offers:

my $iFmt = $oBook->{Worksheet}[0]->{Cells}[0][0]->{FormatNo};

• Full disclosure: I have not used this, for your purpose or any other.
• CPAN or perldoc module::name will often provide information like this.

Afterthought: The module's author is a regular here; perhaps he'll point you in the right direction if the above is misguided.