use XML::Rules;

my $parser = XML::Rules->new(
	style => 'filter',
	ident => '  ',
	rules => {
		_default => 'as array trim',
		elt => sub {delete $_[1]->{_content};$_[0] =>$_[1]},
		'elt_att,selt_att' => sub {$_[0] => $_[1]->{att}},
	}
);

$parser->filter(\*DATA);

__DATA__
<doc>
  <elt elt_class="class1">
    <subelt subelt_class="sclass1">   <content id="content1"/></subelt>
    <elt_att att="elt_att1"></elt_att>
  </elt>
  <elt elt_class="class2">
    <subelt subelt_class="sclass2"><content id="content2"/></subelt>
    <selt_att att="selt_att1"></selt_att>
  </elt>
  <elt elt_class="class3">
    <subelt subelt_class="sclass3"><content id="content3"/></subelt>
    <elt_att att="elt_att1"></elt_att>
  </elt>
</doc>