Which hair service typically requires the use of a perm rod?

The service that typically requires the use of a perm rod is permanent waving. This technique involves wrapping sections of hair around perm rods to create waves or curls in the hair structure. The chemical solution used during permanent waving alters the hair's texture and allows it to maintain the newly formed curls or waves.

In contrast, blow drying does not utilize perm rods, as it primarily involves using heat to style and set hair in its natural texture. Hair curling can be done using various tools like curling irons or rollers, rather than specifically perm rods, while hair straightening focuses on the opposite effect—smoothing out the hair—thus also not requiring perm rods. Permanent waving is unique in its use of these rods to achieve a long-lasting curl or wave effect, making it the correct answer.