The output was not 1/2, but ½, and it also wasn't 1/4, but ¼.

Let's look at what the documentation for the pack function says:

Takes a LIST of values and converts it into a string using the rules given by the TEMPLATE.

So, the function takes a list of values (in our case, the number 171, or the number 172), and converts it into a string, according to our template. The template "i" says that pack is to take the values and pack them into integers in machine representation. Perl then returns the packed values as a string. If you look at the ASCII chart, you will find that at place 171, there is "½", at least for the character set you use. That's all there is to it.

If you want to read some more information on pack, see the perlpacktut tutorial on pack.