Note that you can only set the environment variables for your child processes and not for your parent process (unless you're on Win32).

So if you're running child processes (like a C compiler) from your Makefile.PL, the following will work:

use Config '%Config';
my $allegro_dir = '/usr/local/bin/';
$ENV{PATH} .= ":" . $allegro_dir;
...
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The following won't do what you want though:

`PATH=$ENV{PATH}:$allegro_dir`;
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because that will launch a child process, which sets the PATH environment variable and then ends. And the changes are gone with the end of the child too.