I haven't used it, but it seems Filesys::DiskUsage could accomplish your goal with one line.

Update: If you want to stick with File::Find, the following closely matches du -s or du -sk on my system:

use strict;
use warnings;
use File::Find;
my $dir = shift;
my $k   = shift || 0;
die "usage: $0 <DIR> [-k]\n"   unless -d $dir;

my $size;
find sub {
   next   if /\.svn$/;
   $size  += -s;
}, $dir;
$size = int($size/1024)."K"   if $k;
print "$size\n";
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