use XML::Simple;
use Data::Dumper;

my $xml = XMLin(\*DATA);
print Dumper $xml;
 
$xml->{bar} = [ sort @{ $xml->{bar} } ];
print Dumper $xml;

__DATA__
<foo>
   <bar>4</bar>
   <bar>1</bar>
   <bar>3</bar>
   <bar>2</bar>
   <bar>5</bar>
</foo>