Writing to Exchange is easy as long as you can use the Outlook Object Model. When I used the Outlook Object Model to do live caller-id lookup against a contacts database of about 400 Contact Items on an Exchange (4) server, I found the Outlook Object Model to be abysmally slow for searching through all these contacts. I had (not with Perl, mind you) to turn to true MAPI, which is more or less like SQL without the ASCII command syntax, and you have to manage the result tables yourself. MAPI has the great performance you'd expect from a relational database, but interfacing MAPI to Perl is really hard.

To set the value of a property via the OOM, just assign to that property. As (I guess) this will be a one off job, time is not that important, and you can try to use the Outlook Object Model. Stuff you can't do with the OOM is for example change or preserve sociations between Outlook Items, as there is not much way to get at the MAPI unique item IDs.