See: join, perlretut, perlre and perlreref. Regular expressions are a language in their own right, a major player in Perl's team of tools, and have been adopted by many tools other than Perl. They are well worth becoming familiar with. It's at least worth knowing where to find documentation.

The short version is: | is the alternation character. The match will succeed if any of the alternate matches separated by the | succeed. Join glues a list of strings together using the first string as the glue.

If you are serious about learning and using Perl I strongly recommend you get a copy of the Perl Pocket Reference.