Use Spreadsheet::Read and specify the date format used in your spreadsheet in the call to ReadData():

use Spreadsheet::Read qw/ ReadData rows /;

my $format = 'yyyy-mm-dd'; # or whatever your date/time format is
my $workbook = ReadData('my_file.xlsx', dtfmt => $format);
my @rows = rows( $workbook->[1] );
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Spreadsheet::Read also provides the commandline tool xlscat which extracts all or part of a spreadsheet into a CSV structure, as you appear to want to do. xlscat in turn supports the dtfmt command option. See the docs.

Hope this helps!