Very useful! I found I couldn't write the HYPERLINK function, however, since it was not defined in the hash in Formula.pm. But I figured out the code for HYPERLINK and added it. I didn't have a copy of the Excel Developer's Kit but was able to find the code from the following:
'HYPERLINK' => [ 359, -1, 1, 0 ],
BTW, in case you're wondering, I used HYPERLINK instead of write_url because I wanted to write a URL that was based on a value contained in another cell, and write_url appears to not allow mixing of internal and web addresses.