Ok, here it is the full solution using an in-memory file: 
use strict;
use warnings;
use Path::Tiny;

sub hotpatch {
    my ($self, $module) = @_;
    if ($module eq 'AnyEvent/DNS.pm') {
        for my $inc (@INC) {
            next if ref $inc;
            if (open my $in, '<', path($inc)->child($module)) {
                my $buffer = do { undef $/; <$in> };
                $buffer =~ s/\bsub\s+DOMAIN_PORT\b/sub DOMAIN_PORT () 
+{ 1053 } sub FORMER_DOMAIN_PORT/;
                open my $out, '<', \$buffer
                    or die "Couldn't patch AnyEvent::DNS; your Perl do
+es not support in-memory filehandles";
                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^4: How to redefine a modules private function? by salva
in thread How to redefine a modules private function? by sectokia

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