A Perpetual SIP is simply a SIP set up without specifying a fixed end date — instead of choosing to invest for, say, 5 or 10 years, you leave the duration open-ended, and the SIP keeps running month after month until you actively log in and stop or pause it yourself.
Most SIP application forms ask you to choose between a fixed tenure and a perpetual option, and many long-term investors prefer perpetual, since it removes the risk of forgetting to renew a SIP once a fixed tenure ends — which could otherwise interrupt years of disciplined, uninterrupted investing without you even realising it. It's especially useful for long-horizon goals like retirement, where you genuinely don't know exactly how many years you'll want to keep contributing.
If your goals or cash flow situation ever change, you can always log in and modify, pause, or stop a perpetual SIP just as easily as one with a fixed tenure.