In the computing world, ` (`, backtick or backquote), ' (', single-quote), and " (", double-quote) are all different things, and are used for different purposes. If you can't tell the difference between the three, change your font. The documentation used ` (backtick), because the point is that it runs ls -l, then passes the output of that command to the function parse_dir.

You can find all three documented in great detail in perlop.

(Note, BTW, that while ' is both single-quote and apostrophe, the single-quote meaning is normaly the one that is being referred to when ' is used, except in strange circumstances, or inside of a human-readable string.)