Perianal adenomas are usually a benign tumour that affects older, intact, male dogs. In male dogs these occur in tissues around the rectum that are responsive to androgens (like testosterone) and develop into benign tumours under the effect of these hormones. Unfortunately, when these tumours occur in spayed female dogs they are much more likely to be malignant than not. A malignant version of this tumour is known as a perianal adenocarcinoma. There is no way I know of to tell them apart based on appearance visually. The tumour should be removed and sent to a pathologist for histopathologic examination.

Careful examination of the surrounding tissues might reveal metastasis of the tumour. If your female dog is unspayed, there is a greater chance that the tumour would be benign but it would still be more likely to be malignant than in a male dog.