Net::SSH::Perl::Subsystem::Server is for subsystems (as SFTP), programs that run attached to SSH channels.

For writing servers check Net::Dropbear::SSHd, a fork of the dropbear SSH server with hooks which call back into Perl allowing one to customize its behavior.

Other possibilities would be to wrap libssh which implements the basis for a SSH server, or to extend Net::SSH::Perl with the missing functionality,