Not ready for production. :) With Perl, we can insert code into @INC so that we get ahead of the constant definition:

First, MyBase.pm. This module defines a constant, FOO of 42. But we live in a universe where FOO needs to be 44. Here's the example base:

package MyBase;

use strict;
use warnings;
use Exporter;

our @ISA = qw(Exporter);
our @EXPORT = qw(bar);

sub FOO() {42}

sub bar {
    return FOO();
}

1;
[download]

If someone were to use this module and call bar(), its return value would be 42.

Now I need a module that depends on MyBase.pm. We'll call it MySub.pm. Here's where I can get in front of loading MyBase. To do that I'll insert code into @INC so that use and require get a little modified behavior:

package MySub;


use strict;
use warnings;


sub _filter {
    my $module = $_[1];
    if ($module =~ m/^MyBase/) {
        foreach my $dir (@INC[1..$#INC]) {
            if (-e "$dir/MyBase.pm" && -f _) {
                open FH, '<', "$Bin/lib/MyBase.pm" or die $!;
                last;
            }
        }
        die "Couldn't find MyBase.pm in @INC\n" unless defined *FH;

        return
            \'',
            \*FH,
            sub {
                if (length $_) {
                    $_ =~ s/(sub FOO\(\)\s*\{)(\d+)(\})/${1}44${3}/;
                    return 1;
                }
                else {
                    return 0;
                }
            };
    }

    return ();
}

BEGIN {unshift @INC, \&_filter}
use MyBase;

use Exporter;
our @ISA = qw(Exporter);
our @EXPORT = qw(baz);


sub baz {
    return bar();
}

1;
[download]

Here I have a subroutine named baz which calls MyBase::bar(), which returns the constant stored in MyBase::FOO, which would normally be 42. However, I've inserted a subroutine into @INC named _filter() that looks for the loading of MyBase.pm and replaces the FOO definition with a new one, with a value of 44.

Finally, a small sample app using this:

#!/usr/bin/env perl

use strict;
use warnings;
use FindBin qw($Bin);
use lib "$Bin/lib";

use MySub;

print baz(), "\n";
[download]

If we didn't override FOO in MyBase, this code would print 42. But the approach is successful, and the output is now 44.

Obviously this isn't necessarily robust. A better approach would be to submit a patch to the maintainers of AnyEvent::DNS that makes the hard-coded value configurable. Just by removing the prototype it would become possible to subclass and override, or monkeypatch, for example. But in a pinch, a lot of things are possible. Look at the documentation in require for an explanation of how my approach works.