But hooking into a later sub like a() actually works.

But this kind of patching requires that the code never changes, tho it's arguably safer than a source-filter in disguise.

 
use strict;
use warnings;
use Data::Dump qw/pp dd/;

$|=1;
my $DBG;



BEGIN {
    $DBG=0;

    $\="\n";
    my $old =  $SIG{__WARN__};

    $SIG{__WARN__} = sub {
        my ( $msg  ) = @_ ;

        if ($msg =~ /Constant subroutine a redefined/ ) {
            $SIG{__WARN__} = $old;
            print "REDEFINED", pp caller if $DBG;

            no warnings 'redefine';
            *ALIEN::DOMAIN_PORT = sub() { 666 };

        }
    };

    *ALIEN::a = sub () {"DUMMY"};       # cause redefine warning

}



# used package

package ALIEN;

# ---8<---- snippet from AnyEvent::DNS

sub DOMAIN_PORT() { 53 } 

sub resolver ();

sub a($$) {
   my ($domain, $cb) = @_;

   resolver->resolve ($domain => "a", sub {
      $cb->(map $_->[4], @_);
   });
}

# --->8---- snippet from AnyEvent::DNS

BEGIN {
    print "pre compile" if $DBG;
}

sub test {
    print "Inside test: ", DOMAIN_PORT;
}


test();
[download]

Inside test: 666