How to define an environment variable before loading modules?

I use the AnyEvent::DNS module.

I want to disable IPv6, so that the resolver only makes a request for A record.

AnyEvent::DNS, uses the environment variable $ENV{PERL_ANYEVENT_PROTOCOLS}

But setting the variable does not work; the resolver still sends two requests A and AAAA

Code from AnyEvent::DNS:

our %PROTOCOL; # (ipv4|ipv6) => (1|2), higher numbers are preferred

BEGIN {
   ...;
   my $idx;
   $PROTOCOL{$_} = ++$idx
      for reverse split /\s*,\s*/,
             $ENV{PERL_ANYEVENT_PROTOCOLS} || "ipv4,ipv6";
}

How to define an environment variable before loading modules?

Solution

Since the code that checks the environment variable is in a BEGIN block, it will be run immediately once the Perl compiler reaches it.

When Perl starts compiling your script, it checks for use statements first. So when you use AnyEvent::DNS, Perl loads that module and parses the file. BEGIN blocks are executed at that stage, while code in methods will only be compiled, not executed.

So if you have something like the following, the code you showed above will be run before you even set that variable.

use strict;
use warnings;
use AnyEvent::DNS;

$ENV{PERL_ANYEVENT_PROTOCOLS} = 'ipv4';
...

There are two ways you can circumvent that.

You can put the assignment in your own BEGIN block before you load AnyEvent::DNS. That way it will be set first.

use strict;
use warnings;

BEGIN {
    $ENV{PERL_ANYEVENT_PROTOCOLS} = 'ipv4';
}

use AnyEvent::DNS;

Alternatively, you can just call your program with the environment variable set for it from the shell.

$ PERL_ANYEVENT_PROTOCOLS=ipv4 perl resolver.pl

The second one is more portable, in case you later want it to do IPv6 after all.

Read more about BEGIN in perlmod.