use constant MAX_DIR_SIZE => 100 * 1_048_576; # max of 100 MB
use constant UPLOAD_DIR   => "/path/to/upload/dir";

# later

if ( dir_size( UPLOAD_DIR ) + $ENV{CONTENT_LENGTH} > MAX_DIR_SIZE ) {
    # don't allow the upload
}

# later

sub dir_size {
    my $dir = shift;
    my $dir_size = 0;

    opendir DIR, $dir or die "Unable to open $dir: $!";
    while ( readdir DIR ) {
        $dir_size += -s "$dir/$_";
    }
    return $dir_size;
}