print "Path = ", $ENV{PATH}, "\n";
$ENV{PATH} .= ";c:\temp";          # add c:\temp to it
[download]

Not sure how it works under Windows, but under Unix these changes are not global and only survive for the life of your script and its children.

Yes, that's also true under Windows (I always assumed it was, but then I got to wondering, so I just tested it).

When Perl starts up it creates the %ENV associated array, which contains all your environment variables of the calling process (usually the shell). To catch a quick glimpse of the contents, do something like this:

perl -MData::Dumper -we "print Data::Dumper->Dump([\%ENV], ['ENV']);"
[download]

To make permenant changes on an MSWin32 system you need to muck with the registry. A great way to do that is Win32::TieRegistry. Do so, however, at your own risk.

Update:I just remembered that this is a FAQ!, check out "How can I change the environment from within my Perl script?." I remembered because my post seemed familliar, and then I remembered answering this in Q&A.

I'd just like to emphasize that under WinNT and later, "a system environment variable" means much more than just "an environment variable". WinNT/Win2K environment variables can be "system", "user", or "temporary". "system" ones are set for all processes. "user" ones are set when that particular user logs in. "temporary" ones are set with "SET X=Y" or $ENV{X}="Y".

Getting and setting "system" and "user" environment variables is quite easy using Win32::TieRegistry. There are a couple of things to worry about.

First, don't read a variable via $ENV{PATH} if you want to modify it permanently. One of our best programmers did that and it was a huge mess (more on this later). Second, be sure to enable ArrayValues so you don't change the type of the registry value.

For example, say the "system" "PATH" variable is just the standard %SystemRoot%\System32;%SystemRoot% and your "user" "PATH" is C:\Perl\bin. You log in one day, happen to do SET PATH=%PATH%;C:\doom before you run your neat little script. Then $ENV{PATH} will return "C:\\WinNT\\System32;C:\\WinNT;C:\\Perl\\bin;C:\\doom". Then you update the "system" "PATH" and the next time you log in your PATH is C:\WinNT\System32;C:\WinNT;C:\Perl\bin;C:\doom;C:\Perl\bin. Note the duplicate and how your temporary change has now become permanent.

Update: I have posted a nice little script I have that demonstrates how to do this correctly and also can fix the above mistakes automatically for you.

Note that changes to "system" environment variables via the registry do not take full effect until you reboot. You can make them take almost full effect by properly diddling (that is the technical term for it) with the "Environment" tab of the "System" control panel applet and logging out and back in again.

All you need to do is:

use Env qw(PATH SYSTEMROOT); # or whatever...
[download]

All the info can be found here.

You could also use the built-in pseudohash %ENV like so:

perl -e"print $ENV{PATH}"
[download]

UPDATE: Some day I will snatch the pebble from Fastolfe's hand... ++ to him for beating me to it.