It is generally accepted that it is permissible to open packaging in order to get access to food or other products that are needed for Shabbat. What is the source for this leniency?
A person may break a barrel on Shabbat in order to eat gerogerot (=dried figs) from it, provided he does not intend to make a vessel. And one may not perforate the plug of a barrel to extract wine from it; rather, one must remove the plug entirely to avoid creating a new opening for the barrel. This is the statement of Rabbi Yehuda. And the Rabbis permit puncturing the plug, but they too restrict this leniency and say that one may not perforate the plug of the barrel on its side.
There is a dispute with regard to the rationale for the leniency that allows the barrel to be broken on Shabbat. Rashi explains that it is based on the fact that breaking a barrel is a destructive act that does not constitute a violation of Torah law. The Ran and other commentaries add that destructive acts are only permitted ab initio in cases of this kind, where one performs the act to fulfill a Shabbat need. As noted in the Mishna, one important exception is a situation where opening the package will create a vessel, e.g., where it will now be used for storage. This would be considered a creative act, which is forbidden on Shabbat.
Regarding the discussion about dried figs it should be noted that the Hebrew term usually used for dried figs that were pressed together is deveilim. By using the term gerogerot, the Mishna teaches that even if the figs were only somewhat attached due to their having been packed tightly together, it is permitted to bring a utensil to cut them apart and to open the barrel (Adderet Eliyahu).