Regression.pm is a multivariate linear regression package.
That is, it estimates the c coefficients for a line-fit of the type

      y= c(0)*x(0) + c(1)*x1 + c(2)*x2 + ... + c(k)*xk

given a data set of N observations, each with k independent x variables
and one y variable. Naturally, N must be greater than k---and preferably
considerably greater. Any reasonable undergraduate statistics book will
explain what a regression is. Most of the time, the user will provide a
constant ('1') as x(0) for each observation in order to allow the
regression package to fit an intercept.