There may be some Devel::* modules or some other trickery that could prevent inlining from the outside, but I'm currently unaware of any such modules, though I would be interested to learn about such things.

A relatively simple solution would be to tie AnyEvent::DNS stash, so that the definition of DOMAIN_PORT could be replaced by a custom one:

#!/usr/bin/perl

use strict;
use warnings;
use feature 'say';
use Data::Dumper;
use Tie::Hash;

package Tie::Hash::Mine {
    BEGIN { our @ISA = qw(Tie::StdHash) };
    sub STORE {
        warn "Setting AnyEvent::DNS::$_[1]";
        $_[0]->SUPER::STORE($_[1],
                            ($_[1] eq 'DOMAIN_PORT') ? sub () { 1053 }
+ : $_[2])
    }
};

BEGIN {
    tie %AnyEvent::DNS::, 'Tie::Hash::Mine';
}

#use AnyEvent::DNS; <-- uncommenting this results in a segmentation fa
+ult!
BEGIN {
    package AnyEvent::DNS {
        sub ONE () { 1 }
        sub DOMAIN_PORT () { 53 }
    }
}

say Dumper \%AnyEvent::DNS::;
say Dumper tied(%AnyEvent::DNS::);
[download]

... unfortunately, it seems that tieing a stash doesn't work at all :-(