I wouldn't use Perl; there are tools available to do this, and any POSIX compliant system will have them.

  $ sort file1 > file1.sorted
  $ sort file2 > file2.sorted
  $ comm -12 file1.sorted file2.sorted
[download]

In bash (in several other shells probably as well, but I don't know them well enough), you can even do it as a one-liner, without the use of temporary files (*):

  $ comm -12 <(sort file1) <(sort file2)
[download]

(*) Well, sort may use temporary files if the file to be sorted cannot be held in memory, but it'll clean up afterwards.