I know I should use hashes and know how to split element-by-element but can't figure this one out.

Figure what out, exactly (be specific)?

Anyway, its FAQ

$ perldoc -q duplicate

perlfaq4 How can I remove duplicate elements from a list or array?

$ perldoc -q substring

perlfaq4 How can I count the number of occurrences of a substring within a string?

Also see perlintro#Regular expressions the Simple matching paragraph