What are the nitty-gritties for extensions of perl programs?

1. Technically, the ".pl" extension must be used for Perl libraries, not for executable Perl programs.
2. For executable, you should either specify ".plx" or no file extension at all if  OS allows it.
3. Perl modules must use the ".pm" file extension.

It is good coding paradigm to use only alphanumeric characters and underscores in Perl script file-names, and to start those file-names with a letter (or underscore), similar to how you would start variable names.