I can't help you (your problem is ill defined and I am not sure your variable(s) follow Poisson distribution to which Poisson regression would be applied in lieu of normal multivariable linear regression). Frome has done a lot of work on Poisson regressions here is his web page: http://www.csm.ornl.gov/~frome/
There are a number of articles on the use of Poisson regressions and the use of ML (most likely) instead of least mean squares.