Where to put it? That's a question of how you want to organize it. My scripts live in several folders, depending on which project or topic they belong to...

The .pl extension, I recommend you to keep it, because that's Windows' way to know it's a Perl script. (AFAIK, the installers of both Strawberry and ActivePerl associate it with the Perl interpreter.) On Unix/Linux/Os X, again, it would mostly depend on your preferences.