Pirenzepine isn't used in the United States because it is not approved by the FDA. In studies that have been done, a gel is applied to the eye daily.
Pirenzepine has been found to reduce the rate of myopic progression by about 50% in a study by Siatkowski. The actual site of action where pirenzepine has its affect has not been determined, but several studies have indicated the action may be in the sclera. The studies that have been done are not well controlled - for example the second year of treatment is limited to those who wished to continue with the study, presumably selecting a higher percentage of people where the drug was felt to be working.
Pirenzepine is termed a selective muscarinic receptor M1-antagonist. Although it blocks action where the receptor is located, it is less active in the muscarinic receptors of the pupil and ciliary body than atropine, another common muscarinic antagonist. Thus it doesn't dilate the pupil and cause loss of ability to focus as much as atropine.
Muscarinic receptors are part of the autonomic nervous system. If you want a general outline of the sympathetic and parasympathetic nervous system, collectively called the autonomic nervous system, here is a link to a site (Merck): Autonomic Nervous System
Pirenzepine has been used for the treatment of gastric ulcers.
Ocular irritation is reported in a few subjects.
Pirenzepine is not available in the United States due to the FDA.