use warnings;
use strict;
use XML::Twig;

my $xml=<<'XML';
<Root>
  <Parent>
    <child1>Child 1</child1>
    <child2>Child 2</child2>


    <child3>Child 3</child3>
    
    <ufo> Ufo there!</ufo>
  </Parent>
</Root>
XML


my $twig= new XML::Twig(
                          pretty_print => 'indented',
                          twig_handlers => { '/Root/Parent/*' => \&field },
);

$twig->parse( $xml);

sub field
  { my( $twig, $field)= @_;
    return unless $field->gi() =~ /^child/i;
    $field->print;  #OR print $field->text();
 }

#OUTPUT
#
#    <child1>Child 1</child1>
#    <child2>Child 2</child2>
#    <child3>Child 3</child3>