Corion observed that this can be done using only pure Perl. Which is right but somehow escaped me. So here it is in pure Perl. You still need to execute the script via exec env2.pl.

#!/usr/bin/perl

# setenv from within perl to effect parent shell
# actually parent should be more appropriately
# called "adopted" or "step-parent", re execl
# there is one little requirement:
# you should run this program via shell's exec:
#    % exec <scriptname>
#
# this is pure-Perl implementation after Corion's
# comment.

# set env
$ENV{test} = 'TURE';

# ...

# this should be the last statement in your script
# because nothing else will be executed after that
# not even the 'or die ...'
die "no shell!" unless defined($ENV{SHELL});
exec($ENV{SHELL});

__END__
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Your trick only works interactively because you need to exec a NEW follow up shell, waiting for human input.

This means this illusion won't work in a non interactive script. All code after the exec will be ignored.

On a more serious node, the exec() is a safe (Edit: not safe as in safety and security) way to achieve what the question asked as my use-case demonstrated. It is true that unless one executes the "trick" from an interactive shell any command past that exec will not run as KurtZ whines rightly points out. For example as a cron job.

I don't see it as a "trick" because this is what exec() was intended to do in the first place. It is not exploiting any of its features=bugs, it is not using it in a heads-down-feet-up kind of way.

And definetely the result is not an illusion because it's there. You get your environment modified albeit within a brand new shell which inherits from the parent shell. It inherits env variables and even opened file descriptors. For example:

exec 3> /tmp/out
echo 'before exec' >&3
exec env2.pl
echo 'after exec' >&3
exec 3>&-
cat /tmp/out
   before exec
   after exec
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So, not a trick, not an illusion but limited (and what isn't) to interactive shells.

For non-interactive use, e.g. a cron-job one can go with corion's Re: setenv in perl. And if only 1 env-variable needs to be set up, then the eval can be avoided by using:

export test=`perl -e 'print 'TURE'`
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Lastly, if the scenario is to run a shell command which reads data from the environment and having a perl script to calculate this data and export it to the environment then why not let perl calculate AND spawn the command because any system() call inherits perl's environment vars, like so:

#!/usr/bin/env perl

$ENV{test} = 'TURE'; # calculate
my @cmd = ('command-exec', 'args');
system(@cmd); # spawn

# simple demo:
system('echo $test'); # prints what
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bw, bliako

The same can be achieved with a shell alias or function calling a Perl function and exporting the returned variable value pairs.

with less code and side effects

Nope! Not in a Quantum Computer running UNIX. There, in a parallel universe my "trick"'s "illusion" will warp into reality.

That's kind of Schrödinger's BS, right?

You need to taste it to find out it's not chocolate! ;P

Cheers Rolf
_{(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery FootballPerl is like chess, only without the dice}

Clever. Here's what happens.

When you run Perl with exec, it replaces the shell in the same process. This is the key; just running Perl as it is usually done will not have this effect, which is why you got so many "It can't be done" responses. Because environment variables live in a process, the Perl script sees the same environment the shell did.

When you then exec the shell after modifying the environment, the new copy of the shell, still running in the same process, sees those modifications.

I do have a couple observations to add.

This approach only works if you have control over how your Perl script is run.

Unless the Perl script is in the PATH, I found I needed to explicitly specify the directory: ./env.pl

If you do it this way, you do not need to use Inline::C. Simply modifying %ENV will work:

#!/usr/bin/env perl

use 5.008;

use strict;
use warnings;

$ENV{test} = 'TURE';

exec $ENV{SHELL}
    or die "Failed to exec shell '$ENV{SHELL}'\n";
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works just as well:

$ echo "PID=$$; test='$test'"
PID=87533; test=''
$ exec ./exec.pl 
$ echo "PID=$$; test='$test'"
PID=87533; test='TURE'
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I was trying to follow your post as I think triangulating this question with C makes complete sense. Have I missed a step?

$ touch 1.bliako.pl
$ chmod +x 1.bliako.pl
$ gedit 1.bliako.pl &
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I paste in your code and set it to execute. First I need Inline.pm:

try installing Inline::C