How do I get the full path to a Perl script that is executing?

There are a few ways:

• $0 is the currently executing script as provided by POSIX, relative to the current working directory if the script is at or below the CWD
• Additionally, cwd(), getcwd() and abs_path() are provided by the Cwd module and tell you where the script is being run from
• The module FindBin provides the $Bin & $RealBin variables that usually are the path to the executing script; this module also provides $Script & $RealScript that are the name of the script
• __FILE__ is the actual file that the Perl interpreter deals with during compilation, including its full path.

I’ve seen the first three ($0, the Cwd module and the FindBin module) fail under mod_perl spectacularly, producing worthless output such as '.' or an empty string. In such environments, I use __FILE__ and get the path from that using the File::Basename module:

use File::Basename;
my $dirname = dirname(__FILE__);