#!perl
use strict;
use XML::Twig;
use Time::Piece;

#open my $IN ,'<','origfile.xml'   or die "$!";
open my $OUT,'>','newfile.xml' or die "$!";
my $twig  = XML::Twig->new(
  twig_roots => { 'label_a' => \&label_a }, # process label_a blocks
  twig_print_outside_roots => $OUT, # print rest
); 
$twig->set_pretty_print('indented');
$twig->parse(\*DATA); # or $IN

# process
sub label_a {
  my ($twig,$e) = @_;
  my $timea = $e->att('timea');
  my $id    = $e->att('id');
  
  my $t = Time::Piece->strptime($timea,'%Y%m%d%H%M%S %z');
  my $hr  = $t->hour;
  my $day = $t->day;
  my $keep = 0;
 
  $keep=1 if (( $id =~ /match this/ )
           && ( $hr >=18 && $hr <=20 )
           && ( $day =~ /Mon|Tue|Wed/i ));  
  
  # $keep=1 if ... another condition
  
  if ($keep == 1) {
    $twig->flush($OUT); # save
  } else {
    $twig->purge(); # discard
    print STDOUT $t->strftime." $id $hr $day skipped\n";
  }
}
__DATA__

##</code><code>##

<?xml version="1.0" encoding="ISO-8859-1"?>
<label_x data1="somevalue" data2="someothervalue" data3="anothervalue">
	<label_y y="somevalue">
		<label_z>a value</label_z>
		<label_z>a value</label_z>
		<label_z>a value</label_z>
		<label_z>a value</label_z>
	</label_y>
	<label_y y="somevalue">
		<label_z>a value</label_z>
		<label_z>a value</label_z>
		<label_z>a value</label_z>
		<label_z>a value</label_z>
	</label_y>
	<label_a timea="20140623203000 -0400" timeb="20140623210000 -0400" id="must_match_this">
		<label_b b="data">data_of_variable_number_of_lines_and_indentations</label_b>
		<label_b b="more_data">data_of_variable_number_of_lines_and_indentations</label_b>
		<label_c>
			<label_d>Some_data_may_be_indented_further</label_d>
		</label_c>
		<label_b b="still_more_data">data_of_variable_number_of_lines_and_indentations</label_b>
	</label_a>
	<label_a timea="20140623210000 -0400" timeb="20140623220000 -0400" id="must_match_this">
		<label_b b="data">data_of_variable_number_of_lines_and_indentations</label_b>
		<label_b b="more_data">data_of_variable_number_of_lines_and_indentations</label_b>
		<label_c>
			<label_d>Some_data_may_be_indented_further</label_d>
		</label_c>
		<label_b b="still_more_data">data_of_variable_number_of_lines_and_indentations</label_b>
	</label_a>
</label_x>