use XML::Twig;

  my $t= XML::Twig->new( twig_handlers => 
                           {   'settings' => \&file,
                                        		}
                      
 )  ->parsefile( 'sample.xml');

sub file{
	my($t, $settings)=@_;
	
	foreach($settings->att('key')){
	if($settings->att('key') eq 'a'){
		$a = $settings->att('value');
		
	}
	if($settings->att('key') eq 'b'){
		$b = $settings->att('value');	
	}
	}
}