As a significant example of why flags can be messy, check this particularly thorny source code.

I needed to use that module and I found out that it was faulty. I tried to fix it, but its logic eluded me. I contacted the author, who didn't do anything, and therefore I wrote my own parser using a rather different approach.

This module is supposed to parse chess games. The problem in this code is that it skips the last game in each file it parses. I was able to spot where the problem is. As for fixing it using the same labyrinth of flags system, I simply couldn't.

##
## WARNING! broken code ahead!
##

  my $FLAG=1;
  my $GAME=0;

##
##  ... snip
##

sub ReadGame
{
  my $self=shift;
  my $continue=1;
  $self->_init();
  do
   {
    if ( $continue == 2 ) { $continue = 0 } 
    if ( $LINE =~/\[(.*)\]/ ) 
     {
      my $line=$1;
      my ($key,$val,$junk)=split(/"/,$line);
      $key=~s/ $//g,
      $self->{$key}=$val;
      $GAME=0;
     }
    elsif ( $LINE !~/^$/)
     {
        $self->{Game}.=$LINE; 
        $GAME=1;
     }
    elsif ( $GAME == 1 ) 
     {
        $FLAG=0;  
     }
    $LINE=<FICPGN>;
    if ( eof(FICPGN) && $continue == 1  ) { $continue = 2  }
            $LINE=~s/\r//;
   }
  while ( $FLAG==1 );
  return ( $continue ) ;
 }
[download]

This code uses not one but THREE flags, transforming a would-be OOP module into a peculiar example of spaghetti code.

 _  _ _  _  
(_|| | |(_|><
 _|
[download]