In my quest to make my self-contained example as brief as possible, I neglected to point out that the $row variable comes from an array under source control, and not from user input. All the user can do is press a button. (Yes, technically they could edit the page and make up their own button value, but if that value isn't in the array, nothing is executed.)
An earlier version of my code did use fork/exec, but it didn't work as expected under Mojolicious (Mojo waited for the child to finish), so I moved on to other approaches.
Alex / talexb / Toronto
Thanks PJ. We owe you so much. Groklaw -- RIP -- 2003 to 2013.