What does HCL mean when used as a dosage? example: Percocet 50 HCL?

There is no Percocet 50 and you shouldnt see it after the mg but in the drug formula name. Like Kaismama said, it is just part of the chemical name, oxycodone hydrochloride , hydromorphone hydrochloride, meperidine hydrochloride, methadone hydrochloride, etc. There is also an oxycodone pectinate which is a little different chemical form of the same drug, just like codeine phosphate and codeine sulfate or morphine sulfate and morphine hydrochloride. With codeine and morphine, the sulfate form is most commonly used in the US and the other forms used in Canada. With oxycodone, the hydrochloride form is used in the US but the pectinate form is used in Australia

It means nothing, its just the full name of the drug. You won't see it after percocet as that is a brand name. It would be oxycodone hcl. These names after the name of the drug are just chemical names of the salt of that drug.