Mechateich (Cutting) - Breaking Down the 39 Melachot

If you have a bag of food on Shabbat and the  company puts in a perforated line to allow you   to easily open up the bag, are you allowed  to tear on Shabbat along that perforated   line? Or let's say you have an unopened box of  tissues. Are you allowed to tear open the top,   which will tear along the perforated lines  and create a nice opening? One of the Melachot   of Shabbat is the Melacha of Mechateich, which  means cutting in a measured or exacting way.   So if ever you're tearing along perforated  lines and you're doing so in order to   create a measured and a pretty cut,  then it is not allowed. Therefore,   you are not allowed to open up a new tissue box  because cutting along the perforated lines will   create a measured cut. However, if you have a bag  and it has perforated lines, and those are really   just to make it easier to open up the bag, you  are allowed to tear along the perforated lines.