Set system vairables on one-off builds

Basically what I want to do is allow my users to log into their bamboo site, fire off a plan but be able to pass options to the plan that are specific to the user needs. (Not the set of variables that are already defined within bamboo). So for example, the user launches the Plan passing it some value.. that value ultimately gets used when the build script is called. So if I'm using PERL, it's passed into the PERL script as a parameter and the build acts accordingly.

The idea behind it is that at night when the build runs, it will fully build everything we are working on. During the day, if a developer just wants a certain part of the solution they can pass a parameter stating what they want and the build process will create the correct sub solution.