What should I use for a Perl script's shebang line?

If you have to hard code #!, use #!/usr/bin/env perl. Why? What you want is for the Perl program to run with the user's preferred Perl. That's going to be the first on in their PATH. #!perl doesn't do what I mean, it doesn't search the user's PATH, #!/usr/bin/env perl is how you pull that off. /usr/bin/env will always be there on Unix systems.

If the user is using Windows, as others have pointed out, it doesn't matter. Windows doesn't use #! it uses file extension associations. Make sure your program is called foo.pl or something and it'll work. But include the #! line anyway as some utilities and editors make use of it.

If you're shipping code, let the installer take care of it. Both MakeMaker/Makefile.PL and Module::Build/Build.PL will change your #! line to match the perl the user used to install with. They will take care of this problem for you.

If you are installing code for your own production use, you should use the full path to a particular copy of perl. Which copy of perl? One specific to your project. Does this mean you need to compile perl for every project? No, you can make a symlink. Project foo might have /usr/local/bin/fooperl point at /usr/bin/perl5.18. Use #!/usr/local/bin/fooperl. Now if you decide to upgrade perl you can do it per project by changing the symlink.

If you are running CGI via Apache on Windows, the SHEBANG IS USED. You will need the fullpath to perl.

A Windows she-bang (deduced from the perl.exe bit) seems irrelevant since your (ahem) "shell" probably does not even parse it (correct me if I am wrong, could have been changed lately).

Some command line flags may still be picked up by Perl itself though (according to this thread).

1. As ChristopheD noted, I can confirm from practice (ActivePerl on XP) that the shebang line is not really necessary on Windows.

   A shebang line tells a Unix shell which interpreter to pass the script to.

   On Windows, the program to pass the script to will be determined by associations based on the extension.

2. On Unix, the third option (full path to perl executable) is best.

   And yes, you can use ".." in theory (shell doesn't care) but you should not really use relative path - you never know what your current working directory when executing a script will be.

If you're developing in Unix using Perl and you use "perlbrew" to easily switch between different versions of Perl, then the "#!/usr/bin/env perl" shebang line works well.

I originally had the first 2 characters in the shebang line reversed. Just fixed/edited that.