Pure API calls would probably be 20-30 lines long, looping over all files in all subdirectories with the FindNextFile api call. If you can be sure that the Windows Scripting is installed (it is on Win98+ & Win2K+, and you can install Windows Script Host on 95 or NT), try using something like this:

#!perl
use warnings;
use strict;
use Win32::OLE;

my $fs = Win32::OLE->CreateObject('Scripting.FileSystemObject');

my $dir = 'c:/s';
my $dir_obj = $fs->GetFolder($dir);
print $dir_obj->Size(), " bytes used\n";

__END__
Output:
38521553 bytes used
[download]

HTH