A hired worker is allowed to snack but only while he's on the job; he's not allowed to pack a box lunch to take home. The reason for this is fairly obvious: just because he's allowed to eat on the job does not entitle him to take food home. In fact, doing so is stealing.
The Torah specifically prohibits this case in addition to the general ban on stealing because a person would tend to justify such an action. “I can eat these grapes at work if I want,” one might reason, “so what difference does it make if I take some home?” (This is equally true today when people justify taking home office supplies.)
This mitzvah applies in all times and places. It is discussed in the Talmud in tractate Baba Metzia, in the major analysis of this topic that runs from pages 87a-93b. It is codified in the Shulchan Aruch in Choshen Mishpat 337. This mitzvah is #268 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #187 of the 194 negative mitzvos that can be observed today as listed in the Chofetz Chaim’s Sefer HaMitzvos HaKatzar.