The key difference between physical and cyber perimeters is visibility.
To augment physical perimeter limitations, surveillance cameras (probably with video analytic to detect intruder) and guard patrol are required.
For cyber perimeter, threat actors need to understand what are behind the Internet-facing entry point (web, remote login etc.) in order to reach the internal cyber assets. Their first step is to conduct reconnaissance. See Lockheed Martin, the Cyber Kill Chain® framework.
Organizations nowadays must have a web presence in doing business. The hard part is to minimize the cyber footprint. It’s a matter how well the Internet-facing entry points are configured per best practices (least privileges, exclusion from search engine, scrutinize data input, enforce server-side logic etc.) and sustaining the protection (security patches, version upgrade, hot fixes etc.). Further, regular validation via black box, white box penetration tests are necessary for assurance.