Perianal fistulas usually occur after a perianal abscess has resolved.
They present as a small discharging sore or pimple just near the anus on the perineum which doesn't heal fully. They can block up and form another abscess. They are a small tunnel between the skin and a small opening halfway up the anal canal.
They are managed by laying the fistula open, or closing the internal opening in the anal canal. Sometimes this is difficult, or takes some time.