#!perl
use strict;
use XML::Twig;
my $xml = do { local $/; <DATA> };
my $twig = new XML::Twig(
  twig_handlers =>{ class => \&class},
);
$twig->parse($xml);

sub class {
  my ($t,$elt) = @_;
  
  for my $shape ($elt->children){
    $shape->set_att('class',$elt->att('id'));
    $shape->print;
    print "\n";
  }
}


__DATA__
<data>
<class id="abc">
  <polygon coordinates=".."/>
  <polygon coordinates=".."/>
  <rectangle coordinates=".."/>
</class>

<class id="def">
  <polygon coordinates=".."/>
  <rectangle coordinates=".."/>
  <rectangle coordinates=".."/>
</class>
</data>