Which of these is better or faster to use as the shebang line for a Perl script?
#! perl
#! perl.exe
#! fullpath/perl(/perl.exe)
#! partialpath/perl(/perl.exe)
And, when using #!perl, when it works on a particular system, how do I find out in the script which perl interpreter I'm using so I can put that one into the shebang line?
And, if using a /path/path/perl, are "*" or "..." allowed to be used for the folders?
A Windows (deducted from the perl.exe bit) she-bang seems irrelevant since your (ahum) '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).
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.
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.
And, when using "#! perl", when it works on a particular system, what is the print() for showing the full path to perl.exe, that could be included into the Shebang Line ?
Well, if you're using the print statement you are already executing perl code, so...
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.
Otherwise, you should ship your program with some sort of installer. 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.
This is one of the things which I dislike about Perl.
On Windows, if you are using ActiveState Perl at least, if the file ends with .pl then the Windows registry will run the Perl interpreter for you, regardless of the shebang line. On Cygwin, I am not sure why but #! perl works too. On Unix you should put the full path to your Perl executable in the shebang line. Schwern's idea of using env is convenient, but has some danger, as I pointed out in a comment.
This is why I suggest to you that the best solution is to package your Perl scripts as CPAN modules. CPAN installers like Module::Build will then change the shebang line to the full path to your Perl interpreter. (I am not sure whether Schwern's installer, ExtUtils::MakeMaker, does this or uses env, since I don't use it.)