Running cron without setting a cron job on web server I know that in Drupal 7 Poormanscron is part of the core, but to have cron running I need the site must be visited regurarly and it's not my case.
I can't set up a cron job on web server to periodically run cron.

Is there any way to run cron periodically without setting a cron job on web server?
Yes indeed, Drupal exposes a URL to invoke cron, e.g.

http://www.example./com/cron.php?cron_key=ka_88ikb7ZbkszCQo6_GtZclwUsUsskfH54oOJqqBcE


Which you can call from anywhere, using whatever method you like (pingdom jumps to mind, for example). Obviously if your site is an intranet or something you'll need to work out how to route that request either from the internal or external network, but Drupal has you covered however you decide/need to do it.

You can find your external cron URL by visiting the status report at /admin/reports/status.

Note that if you have an advanced cron module like Elysia cron, your external URL may be different. Check the documentation for the specific module to be sure.