Thanks jmcnamara, I overlooked it in the documentation.
In Spreadsheet::WriteExcel, search for the set_num_format under the CELL FORMATTING section. You find this...

$format04->set_num_format('$0.00');
$worksheet->write(3,  0, 49.99,     $format04);    # $49.99

$format05->set_num_format('£0.00');
$worksheet->write(4,  0, 49.99,     $format05);    # £49.99

$format06->set_num_format('¥0.00');
$worksheet->write(5,  0, 49.99,     $format06);    # ¥49.99
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regexes