I had to exercise due diligence to design and implement the code right as it had to run for at least a full-week non-stop to complete the job.
An entire week? That doesn't sound right. Are you sure Perl was the best choice for the job? Could an extra couple hours of optimizing have saved a couple days of execution time?
Is it reasonable to turn away such requests or pursue them with vigor?
Just make sure they know the disadvantages. Security, reliability, maintainability, and many other things are reduced by rushed development. Sometimes it's worth it, sometimes it's not.