Implementations

File::Which searches the directories of the user's "PATH" (the current implementation uses File::Spec#path to determine the correct "PATH"), looking for executable files having the name specified as a parameter to "which". Under Win32 systems, which do not have a notion of directly executable files, but uses special extensions such as ".exe" and ".bat" to identify them, "File::Which" takes extra steps to assure that you will find the correct file (so for example, you might be searching for "perl", it'll try perl.exe, perl.bat, etc.)

Linux, *BSD and other UNIXes

There should not be any surprises here. The current directory will not be searched unless it is explicitly added to the path.

Modern Windows (including NT, XP, Vista, 7, 8, 10 etc)

Windows NT has a special environment variable called "PATHEXT", which is used by the shell to look for executable files. Usually, it will contain a list in the form ".EXE;.BAT;.COM;.JS;.VBS" etc. If "File::Which" finds such an environment variable, it parses the list and uses it as the different extensions.

Cygwin

Cygwin provides a Unix-like environment for Microsoft Windows users. In most ways it works like other Unix and Unix-like environments, but in a few key aspects it works like Windows. As with other Unix environments, the current directory is not included in the search unless it is explicitly included in the search path. Like on Windows, files with ".EXE" or <.BAT> extensions will be discovered even if they are not part of the query. ".COM" or extensions specified using the "PATHEXT" environment variable will NOT be discovered without the fully qualified name, however.

Windows ME, 98, 95, MS-DOS, OS/2

This set of operating systems don't have the "PATHEXT" variable, and usually you will find executable files there with the extensions ".exe", ".bat" and (less likely) ".com". "File::Which" uses this hardcoded list if it's running under Win32 but does not find a "PATHEXT" variable.

As of 2015 none of these platforms are tested frequently (or perhaps ever), but the current maintainer is determined not to intentionally remove support for older operating systems.

VMS

Same case as Windows 9x: uses ".exe" and ".com" (in that order).

As of 2015 the current maintainer does not test on VMS, and is in fact not certain it has ever been tested on VMS. If this platform is important to you and you can help me verify and or support it on that platform please contact me.

which

 my $path = which $short_exe_name;
 my @paths = which $short_exe_name;

Exported by default.

$short_exe_name is the name used in the shell to call the program (for example, "perl").

If it finds an executable with the name you specified, "which()" will return the absolute path leading to this executable (for example, /usr/bin/perl or C:\Perl\Bin\perl.exe).

If it does not find the executable, it returns "undef".

If "which()" is called in list context, it will return all the matches.

where

 my @paths = where $short_exe_name;

Not exported by default.

Same as "which" in array context. Similar to the "where" csh built-in command or "which -a" command for platforms that support the "-a" option. Will return an array containing all the path names matching $short_exe_name.

$IMPLICIT_CURRENT_DIR

True if the current directory is included in the search implicitly on whatever platform you are using. Normally the default is reasonable, but on Windows the current directory is included implicitly for older shells like "cmd.exe" and "command.com", but not for newer shells like PowerShell. If you overrule this default, you should ALWAYS localize the variable to the tightest scope possible, since setting this variable from a module can affect other modules. Thus on Windows you can get the correct result if the user is running either "cmd.exe" or PowerShell on Windows you can do this:

 use File::Which qw( which );
 use Shell::Guess;
 
 my $path = do {
   my $is_power = Shell::Guess->running_shell->is_power;
   local $File::Which::IMPLICIT_CURRENT_DIR = !$is_power;
   which 'foo';
 };

For a variety of reasons it is difficult to accurately compute the shell that a user is using, but Shell::Guess makes a reasonable effort.