can't call method on an undefined error in perl script

[veerubiji]

Hi I am getting error while running perl script "can't call method "findnodes" on an undefined value at c:\work\pm.line 17".My script as shown below.

#!/usr/bin/perl
use warnings;
use strict;
use XML::LibXML;
my @xml_files = File::Find::Rule->file->maxdepth(1)->name('*dtc.xml')-
+>in( "c:/file/save");
my $bigXML = XML::LibXML::Document->new( '1.0', 'UTF-8');
my $aggregated; for my $xml_file ( @xml_files ) {

my $doc = XML::LibXML->new->parse_file( $xml_file );

my ( $specNode ) = $doc->findnodes( '//Specification' );

if ( $aggregated ) {
    $aggregated = $specNode;
}
else {
    my @details = $specNode->findnodes( './Definition' );
    $aggregated->addChild( $_ ) foreach @details;
}
}
$bigXML->adoptNode( $aggregated ); 
$bigXML->toFile( 'aggregated_data.xml' );

I Modified above code like that and this time it gives diffrent error like "XML::LibXML::Document::adoptNode()--node is not a blessed SV reference at c:\work\pm.line 17 " help me with this error I didn't see this type of error befor.