This is a great question. (Well, in my mind, any question that I've struggled with is a great question :)

FWIW, here is a working lexer I wrote for use with Parse::Yapp, for a simple OO class-specification/templating language... if you have any questions about it, let me know:

sub Lexer {
  my $parser = shift;

  defined $parser->YYData->{INPUT} or return('',undef);

  if (not defined $parser->YYData->{LINE}) {
    $parser->YYData->{LINE} = 0;
    $parser->YYData->{SLINE} = 0;
    $parser->YYData->{ADDLINES} = 1;
  }
  
  $parser->YYData->{INPUT} =~ s/^[ \t\r]*//;      # remove whitespace
  $parser->YYData->{INPUT} =~ s/^#[^\n]*//;       # remove comments

  for ($parser->YYData->{INPUT}) {
    s/^([\n{}:])// and do {
      return($1,$1);          # character tokens
    };
    s/^(\w+)\b// and do {
      if (exists $modifiers{$1}) {
        return('MODIFIER', $1);
      } elsif (exists $keywords{$1}) {
        return('KEYWORD',$1);
      } else {
        return('IDENT',  $1);
      }
    };
    s/^('(.*?)')// and do {
        return('STRLIT',$2);
    };
    s/^("(.*?)")// and do {
        return('STRLIT',$2);
    };
    s/^(<{(.*?)}>)//s and do {
      my $heredoc = $2;
      $parser->YYData->{ADDLINES} += () = $heredoc =~ m/\n/g;
      return('HEREDOC',$heredoc);   # woohoo i love heredoc's!
    };
  }
}
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