You can do this with write_rich_string() in Excel::Writer::XLSX, the replacement for Spreadsheet::WriteExcel.

    my $bold   = $workbook->add_format( bold   => 1 );
    my $italic = $workbook->add_format( italic => 1 );

    $worksheet->write_rich_string( 'A1',
        'This is ', $bold, 'bold', 
        ' and this is ', $italic, 'italic' );
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