The first argument to the pack and unpack function is not Perl code, but a string. And what is allowed in that string is defined by the documentation for the pack function, which you can find by typing perldoc -f pack in a console window, or by visiting pack, or pack maybe.

Stuff inside of quotes is not seen by Perl as Perl code, but only as a literal value. Just like the difference between the word "And" and the way you concatenate two sentences.