A simple state machine (with information missing):

         INITIAL_STATE  SOME_STATE     FINAL_STATE
         +-----+        +-----+        +-----+
start--->|     |---+--->|     |------->|     |--->accept
         +-----+   |    +-----+        +-----+
                   |       |
                   +-------+
[download]

We can imagine a state machine as a bunch of gotos.

INITIAL_STATE: {
   ...
   goto SOME_STATE;
}

SOME_STATE: {
   ...
   if ( ... ) {
      goto SOME_STATE;
   } else {
      goto FINAL_STATE;
   }
}

FINAL_STATE: {
   ...
}
[download]

We could replace those gotos with sub calls.

sub initial_state {
   ...
   return some_state();
}

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

sub final_state {
   ...
   return;
}

initial_state();
[download]

The stack will keep growing and growing unless we perform tail call elimination, which can be done using goto &SUB

sub initial_state {
   ...
   goto &some_state;
}

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

sub final_state {
   ...
   return;
}

initial_state();
[download]

This is the approach used by the code in question. goto &sub is slow, though. (Slower than a sub call.) I think we could use a loop to speed things up.

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->();
}
[download]

The code in question appears to have some code place to support this. It "returns" the new handler sub (by assigning it to $_[0]->{state}), but it never ends up using it.

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