How do I set a variable to a particular value in a regression file?

If, by regression file, you mean things created by :regression, then you dont. It is presumed to be a complete conversation and all variables come via that conversation without variance.  If, instead, you mean to use sample inputs to validate a pattern, then you can assign values on the sample input, like

#! this is my life $happy=true

Sorry, I wasn’t very clear.  I meant in the :source file that I use to create the regression file. I found that I could do what I wanted by putting :do $variable=value