#!/usr/bin/perl

package test_package;

use strict;
use warnings;

sub new { return bless {}; }

sub test_function { print "Original test_function! $_[1]\n"; }

sub loader {
    my $self         = shift;
    my $override     = shift; # Name of function to override.
    my $new_function = shift; # code ref to install

    my $backup; # keep a code ref of the original here.

    local @_;
    eval {
        no strict 'refs';
        no warnings 'redefine';

        die "Can't override non-existant function $override\n"
            unless exists &{$override};
        
        $backup = \&{$override};
        *{$override} = $new_function;
    };
    
    if ( $@ ) {
        warn "ERROR: $@\n" if ( $@ );
        $backup = undef;
    }

    return $backup;
}

package main;

use strict;
use warnings;

my $counter = 0;

my $f = new test_package();

$f->test_function($counter++); # 1

my $newsub =  sub { print "New output! $_[1]+\n"; 1 };

# Replace function, get backup in return.
my $backup = $f->loader( 'test_package::test_function', $newsub );

$f->test_function($counter++);

# Restore function.
$f->loader( "test_package::test_function", $backup);
$f->test_function($counter++);

my $faked =$f->loader( "test_package::fake_function", $newsub );
warn "Unable to override function" unless $faked;


#$f->fake_function( 'Fake');  # program will die