join($sep, $string1, $string2, $string3)
is the same thing as
$string1 . $sep . $string2 . $sep . $string

In other words, it takes a list and/or array of strings, and combines them into one long string. The first argument is a seperator. It's inserted bewteen every string in the list. The core perl functions (including join) are documented in perlfunc.