If you're wanting to sum the sizes of all files within a directory, this should do it:

my $dir      = '/path/to/dir';
my $dir_size = 0;
my $file;

opendir DIR, $dir or die "Unable to open $dir: $!\n";
$dir_size += -s "$dir/$file" while defined( $file = readdir DIR );
closedir DIR;
print $dir_size;
[download]

This does not descend into subdirectories, though.