HCL - hydrochorlic acid, which is one of the many ancillary ingredients included in the Hulda ClarkLiver Flush protocol.
If you get the Liver Flush Kit from a Hulda Clark authorized supplier, it will include a small bottle of HCL, a diluted form, somewhere around 5% I believe. Clark advises it in an effort to help sterilize some of the various liquids to be consumed during her method for flushing the liver.