use v5.14;
use warnings;


my ($state,$last);

sub initial_state {
    say "*** Initializing";
    return "some_state";
}

sub some_state {
  my $in = shift;
  if ( not $in ) {
    return "some_state";
  } else {
    return "final_state";
  }
}

sub final_state {
  say "*** Finalizing";
  return undef;
}


$state = "initial_state";

my @input = (0,0,0,1);
my $log = 1;
while ( $state ) {
  $last  = $state;
  my $in = shift @input;
  no strict 'refs';
  $state   = $state->($in);
  say "$last \t--($in)-> \t$state" if $log && $state;
}