perl6 -e 'use HTTP::Easy::PSGI; HTTP::Easy::PSGI.new(:port(5000)).handle(sub (%e) { });'
if something is already listening on port 5000, it will print:
[2015-07-15T06:48:11Z] Started HTTP server.
but of course it won't get any requests
Note: It properly dies if trying to bind a low port and no privileges