sub initial_state {
   ...
   return \&some_state;
}

sub some_state {
   ...
   if ( ... ) {
      return \&some_state;
   } else {
      return \&final_state;
   }
}

sub final_state {
   ...
   return undef;
}

my $state = \&initial_state;
while ( $state ) {
   $state = $state->();
}