Here's some code snippets which will set you on the right track:

use Spreadsheet::ParseExcel::Utility qw(ExcelFmt);
...
my $datefmt = 'yyyymmdd';    # ISO 8601
...
if (defined $cell->{Type} && $cell->{Type} eq 'Date') {
    $value = ExcelFmt($datefmt, $cell->{Val});
}
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See Spreadsheet::ParseExcel::Utility for details.