merlyn recommends reserving the .pl extension for "perl libraries", meaning files that contain code to be "included" in other programs. He goes on to say that "experts recommend" using .plx to indicate that the file is a "perl executable" if you either choose to or are required to use an extension.

Any errors in the paraphrasing are my own. Refer to to the footnote at the bottom of page 7 in Learning Perl Objects, References & Modules.