OK, here is my take (using the latest CPAN version of XML::Twig). The only difficult thing here is how to match elements which do _not_ have the vob attribute. I have to look at the XPath spec (Matt?) to figure out how to express it... and then add it to the module. In the meantime I use a coderef to get it (to compute @vob_not I use an anonymous sub that returns true if the element does not have a vob attribute).

#!/bin/perl -w
use strict;
use XML::Twig;

my $twig= new XML::Twig;

$twig->parse( \*DATA);           # build the twig
my $root= $twig->root;           # get the root of the twig (stats)

my @vob_not    = $root->children( sub { my $elt= shift; !$elt->att( 'v
+ob')});
my @vob_none   = $root->children( q{player[@vob="none"]}    );
my @vob_letter = $root->children( q{player[@vob=~/^[a-z]$/]});
   @vob_letter = sort { $a->att( 'vob') cmp $b->att( 'vob') }
                    @vob_letter;
my %letters= map { $_->att( 'vob') => 1 } @vob_letter;

print "No attribute or vob=none\n",
      "-" x 20, "\n";
foreach my $player (@vob_not)
  { printf( "%-20s\n", $player->field( 'name')); }
foreach my $player (@vob_none)
  { printf( "%-20s....none\n", $player->field( 'name')); }


print "\n",
      join( ', ', sort keys %letters), "\n",
      "-" x 20, "\n";
foreach my $player (@vob_letter)
  { printf( "%-20s....%1s\n", $player->field( 'name'), $player->att( '
+vob')); }

__DATA__
<stats><player vob="d" ><name>Houston, Allan</name><g>69</g></player>
<player vob="z" ><name>Sprewell, Latrell</name><g>69</g></player>
<player vob="a" ><name>Ewing, Patrick</name><g>49</g></player>
<player vob="none" ><name >Johnson, Larry</name><g>57</g></player>
<player vob="g" ><name>Camby, Marcus</name><g>48</g></player>
<player vob="d" ><name>Thomas, Kurt</name><g>67</g><ppg>7.9</ppg></pla
+yer>
<player vob="none" ><name >Ward, Charlie</name><g>59</g><ppg>7.5</ppg>
+</player>
<player vob="none" ><name>Wallace, John</name><g>55</g><ppg>6.6</ppg><
+/player>
<player><name>Childs, Chris</name><g>61</g><ppg>5.3</ppg></player>
<player><name>Duncan, Tim</name><g>66</g><ppg>23.1</ppg></player>
<player><name>Robinson, David</name><g>68</g><ppg>17.1</ppg></player>
</stats>
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Update: note that this is not really the most efficient way, XML::Twig will go through the entire list of children for each list it builds), just the easiest to write.