It's "three letters, two digits, followed by a comma, by one or more whitespace characters, and" then... uhm... there is no \i defined for perl regexes. So probably just ".. the word interlock".

split looks for this pattern, and returns the part left of the first match, and between the first and the second match (the 2 at the end of the line limits it to two items).

The join concatenates the first part of the previous expression (that is, the part of the string before the first match) with $date_intlck_rel_res.

(Update: I forgot the digits... johngg++)