How do you recommend differentiating between localized cutaneous melanoma of the perianal skin versus mucosal melanoma of anal canal?  

Although perianal melanomas arise within squamous epithelium, I think it is better to characterize these as mucosal melanomas. I think of it as being similar to melanomas that arise within the cutaneous surface (i.e. squamous epithelium) of the vulva but are still considered to be mucosal melanomas....