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use strict;
use XML::Rules;
...
my $heads = $reader->parsefile( $small_file_name )->{head};
#use Data::Dumper;
#print Dumper($heads);

my $updater = XML::Rules->new(
    style => 'filter',
...
);

$updater->filterfile( $big_file_name, $updated_big_file_name);