The problem is that the

my $e_parent = $t->findnodes($parent);
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doesn't return anything. It does look a bit strange, but I don't know XML::Twig enough to be able to suggest the correct syntax. This is how the solution would look with XML::Rules:

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>
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The rule for the <elt> is necessary because XML::Rules in the filter mode copies everything it parses as is until it encounters a tag with a custom rule so if there was no special rule for <elt> the module would copy the opening <elt> tag, the <subelt> tag with contents and then would process the <elt_att> and be unable to return back to the <elt> to add the results of the <elt_att> rule to the <elt> tag and would create a <elt_att>elt_att1</elt_att> tag.