I'm pretty sure that should work. Are you sure the row height is large enough to fully display the cell?
See: Excel single cell line breaks and Spreadsheet::WriteExcel Newline character
Summary:

my $wb = Spreadsheet::WriteExcel->new('tmp.xls');
my $format = $wb->add_format();
$format->set_text_wrap() ;
my $ws = $wb->add_worksheet();
$ws->write(0, 0, "This is\na multiline cell", $format);
$wb->close();
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