Another way to do this would be to use the File::Recurse module from CPAN. Like so:

use strict;

use File::Recurse;

MAIN: {
    my $dir = $ARGV[0];
    my $size = 0;

    recurse(\&sum_size, $dir, \$size);

    print "Total size: $size\n";

}

sub sum_size
{
    my ($file, $total_size_ref) = @_;

    stat($file);    
    unless (-d _) {
        $$total_size_ref += -s _;
    }

    1;
}
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There's something funky about the above... it produced larger values than crulx's code for a couple of my directories, reason unknown. Seemed fairly accurate, though...