It seems that all you need is a small program that first checks if there's already a copy of itself running, and exits if this is true. And if no copy is running, you call rsync. Something like:

use Fcntl ':all';
my $lock = "/var/lock/whatever";
sysopen my $f, $lock, O_WRONLY | O_CREAT, 0644 or die;
flock $f, LOCK_EX | LOCK_NB or exit;
system 'rsync', ....;
__END__
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