To wrap things up: it works and it doesn't.

The following code runs without errors.

Problem is that the handler is actually called before the redefinition happens. Which means the successful change of the constant (from "Dummy" to 666) is reversed again to 53 ... 

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

$|=1;


BEGIN {

    $\="\n";
    my $old =  $SIG{__WARN__};
    my $new_constant = sub() { 666 };
    *ALIEN::DOMAIN_PORT_NEW = sub() { 666 };

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

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

            print ALIEN::DOMAIN_PORT();
            no warnings "redefine";
            *ALIEN::DOMAIN_PORT_OLD = *ALIEN::DOMAIN_PORT;
            *ALIEN::DOMAIN_PORT = *ALIEN::DOMAIN_PORT_NEW;
            print ALIEN::DOMAIN_PORT();
        }
    };

    *ALIEN::DOMAIN_PORT = sub () {"DUMMY"};

}




package ALIEN;

sub DOMAIN_PORT() {
    53
}




BEGIN {
    print "pre compile";
}

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


test();

[download]

REDEFINED("main", "d:/tmp/pm/patch_constant.pl", 39)
DUMMY
666
pre compile
Inside test: 53

[download]

Cheers Rolf
(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery

In reply to Re^4: How to redefine a modules private function? by LanX
in thread How to redefine a modules private function? by sectokia

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