How can I compute probabilities for a student t distribution when the degrees of freedom is not an integer? I’m using the perl command tprob, but it gives me an error when degrees of freedom is not an integer.

I would not be surprised if the statistical distributions code is not written to handle the case where the degrees of freedom is not an integer.

If you need more advanced statistical capabilities you can use the R integration with WeBWorK. From within that environment you can call any functions available in R, and also load R libraries if they are installed on your server.