On windows, I don't believe it is possible to trace back to the path of the script, and for a very obvious reason: Perl scripts on a win32 machine are executed via a file association: the windows registry contains entries which say that a .pl extension (for example), is to be run by Perl (located at 'c:\perl\bin\perl.exe' for example). So it's not your actual script that is being executed, it is Perl being executed, with your script name passed as an argument to perl.exe. To prove that this is true, you can remove the shebang line from scripts on a windows machine and it will still run (with the exception of CGI scripts, though I'm sure this is an Apache issue, not Perl's).

I know that *nix machines run a program under the script name rather than perl with the script name passed as an argument. I'm going to assume that this is because you are really executing the script as a program, with the shebang line pointing to the interpreter.

So, on Win32, it is not possible to get the script name (perhaps a Win32::xxxx module provides this functionality?). I would much rather avoid OS-specific modules however, and describe a cross-platform method of doing so. Use a status file. :) When your program starts up, you check for the existence of this file. If the file exists, you die with a friendly message. If it doesn't exist, you create the file. Do your work, then delete the file to allow another instance of it to start. Simple. You should probably extend the code to check the last modified date/time of the file (check out stat) and use it to see if that date is older than however long your script takes to run. Do that right, and you won't be stuck if your script is forced to exit before it deletes the file. (ie: someone ctrl+z's the script: the status file will continue to exist, though the script is no longer running. Another instance will never start until you manually delete the file). The example (low-level-code-that-needs-some-add-ons) code:

#!/usr/bin/perl -w
use strict;

my $stat_file = '/path/to/status.txt';

die "Another instance of this program is running!\n"
  if (-e $stat_file);
open F, '>', $stat_file; close F;

# Do all your hard work here
# Nice and boring stuff here... yippee!

unlink $stat_file;
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