Your headline asks for a faster way, but the message asks how you can get the size of a directory other than the one where you are.

Of course, I'm writing on Linux, but the routine should work on all platforms.

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;
}
[download]

Of course, the easy answer, especially if you're interested in subdirectories, too, is:

$size = `du -s -k $dir`;
[download]

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