Well, and now, something that actually works: 
#!/usr/bin/perl

use strict;
use warnings;
use feature 'say';
use Path::Tiny;
use File::Temp qw(tempfile);

sub hotpatch {
    if ($_[1] eq 'AnyEvent/DNS.pm') {
        for my $inc (@INC) {
            next if ref $inc;
            my $fn = path($inc)->child($_[1]);
            if (open my $in, '<', $fn) {
                my ($out) = tempfile(UNLINK => 1);
                while (<$in>) {
                    s/sub\s+DOMAIN_PORT\b/sub DOMAIN_PORT () { 1053 } 
+sub FORMER_DOMAIN_PORT/;
                    print {$out} $_;
                }
                seek($out, 0, 0);
                return $out;
            }
        }
        warn "couldn't patch AnyEvent::DNS";
    }
    return undef;
}


BEGIN { unshift @INC, \&hotpatch }

use AnyEvent::DNS;

BEGIN { @INC = grep not(ref and $_ eq \&hotpatch), @INC }


say AnyEvent::DNS::DOMAIN_PORT();
say AnyEvent::DNS::FORMER_DOMAIN_PORT();

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In reply to Re^2: How to redefine a modules private function? by salva
in thread How to redefine a modules private function? by sectokia

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