If you have XML::PYX installed you can then run this:

pyx pm.xml | perl -n -e 'if( /^Areputation (\d+)/) { $rep+=$1;$posts++
+;} \
END { print "$posts posts, $rep rep, ", sprintf( "%5.2f", $rep/$posts)
+,  " average\n";}'
[download]

And of course you can also use XML::Twig to get the same result:

#!/bin/perl -w

use strict;
use XML::Twig;

my $posts= 0;
my $rep=0;

my $twig= new XML::Twig( TwigRoots => 
            { NODE => sub { my( $t, $node)= @_;
                            $posts++; 
                            $rep +=$_[1]->att( 'reputation'); 
                            $t->purge;
                          }
            });

$twig->parsefile( $ARGV[0]);
print "$posts posts, $rep rep, ", sprintf( "%5.2f", $rep/$posts),  " a
+verage\n";
[download]