Thanks Gloryhack for the pointers and for listing these caveats. Indeed one needs to be careful where and when to apply XMLHTTP requests.
I am not too concerned about loosing backbutton functionality. This side-effect will be limited when one still uses regular page linking for a switch between menu choices and between forms. In fact the XMLHTTP requests can be used beneficial to prevent accidental re-posting of form data when the user hits a backbutton.

The prototype links indeed appear to come close to what I was looking for and I will investigate them further. In particular I am curious to see how they would interact with Perl code back and forth.

I am also grateful for the feedback from the other monks. As usual there's more than one way to accomplish things and the choices and tradeoffs mentioned provide insight.