This error appears when attempting to make an SSH connection using a Perl SSH module, such as Net::SSH:Perl.

Host key for 'server1' has changed

Let's say you are invoking the Perl script as john.doe. In this scenario, on a Linux system, the public certificate in John Doe's known_hosts file (/home/john.doe/.ssh/known_hosts) does not match the public certificate being issued by the target server.

You can simply remove the public certificate from John Doe's known_hosts file, and the error should no longer occur.