use File::Find;

my $client = REST::Client->new( $an_url );
File::Find::find( sub { 
        return unless m/\.xml$/;
        carp "Could not open $File::Find::name!" 
            unless open( my $fh, '<', $File::Find::name )
            ;
        my $doi;
        while ( <$fh> ) { 
            next unless ( $doi ) = m{[^/]*/([^/]*)};
            $client->GET( join( '/', $base, $doi ));
            do_stuff_with_content( $client->responseContent );
        }
        close $fh;
    } => '.'
);