use strict;
use warnings;

my %states = (
    main            => {
        next     => [ qw(intermediate end) ],
        on_enter => sub { print "You entered state main\n" },
    },
    intermediate    => {
        next     => [ qw(intermediate end) ],
        on_enter    => sub { print "stupid side effect\n"; },
    },
    end             => {
        next     => [],
        on_enter    => sub { print "Never called\n" },
    },

);

my $current = $states{main};

while (@{$current->{next}}){
    $current->{on_enter}->();

    print "You can go to the following states from here:\n";
    for (@{$current->{next}}){
        print "\t$_\n";
    }
    my $next = <STDIN>;
    chomp $next;
    if ($states{$next}){
        # go to the next state:
        # TODO: check that this state transition is allowed
        $current = $states{$next};
    } else {
        print "Sorry, don't understand your input, try again please\n";

    }
}