It produces a string that contains all of its arguments, separated by the first one. In this case, it takes all the @files, puts commata between them, and makes that one large string.

If you don't know what something does, you can look it up in the extensive perldoc. Try perldoc -f join on the commandline and see what it tells you. If you're using ActiveState Perl on Windows, you will probably want to find the HTML version of the documentation it ships with and installs. There is hardly any trivial question you cannot answer straight off the docs within a minute.

Makeshifts last the longest.