As we mentioned in Mitzvah #576, the hired worker is allowed to snack on the produce he is reaping or otherwise engaged in processing but he should not do so when he's supposed to be working. When the Torah says not to raise a sickle over the standing grain, it means not to eat when it's “sickle time,” i.e., when he should be working (see Talmud Baba Metzia 87b).
The reason for this mitzvah is that the owner is paying the worker to do a job, not to eat. The owner must be generous enough to allow the worker to eat but the worker must be considerate enough that doing so does not deprive the employer of his labor.
This mitzvah applies in all times and places. It is discussed in the Talmud in tractate Baba Metzia among other laws of employees eating that are found on pages 87a-93b. It is codified in the Shulchan Aruch in Choshen Mishpat 337. This mitzvah is #267 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #186 of the 194 negative mitzvos that can be observed today as listed in the Chofetz Chaim’s Sefer HaMitzvos HaKatzar.