my $dirname = "/usr/local/bin";
my $size = get_dir_size( $dirname );

# Perl Cookbook recipe 9.5
#
sub get_dir_size {
    return unless @_ && -e $_[0] && -d _ && -r _ ;
    my ( $dir ) = @_;
    my ( $file, $size ); 
    opendir DIR, $dir or die( "Could not open directory '$dir'\n" );
    while ( defined ( $file = readdir( DIR ))) {
        $size += -s "$dir/$file";
    }
    closedir DIR;
    return $size;
}

##</code><code>##

$size = `du -s -k $dir`;