#!/usr/bin/perl
use strict;
use warnings;

# set up
srand;
my @one = (1,2,3,4,5,6);
my @two = (2,4,6,8,9,12);

# the value that needs replacing
my $bad = 4;

# figure out where that value is in list one
### my $index = index(join('',@one),$bad); # <- bad code
my $index = 0;
my $found = 0;
foreach my $item (@one) {
	if ($item == $bad) {
		$found++;
		last;
	}
	$index++;
}

die "The value '$bad' was not found in the list\n" unless ($found);

# find elements of list two that aren't already in list one
my $re = join('|',@one);
my @candidates = grep(! /^(?:$re)$/, @two);

# replace unwanted list one value with random selection from list two
$one[$index] = $candidates[rand @candidates];

use Data::Dumper::Simple;
print Dumper(@one);

__END__
@one = (
         1,
         2,
         3,
         8,
         5,
         6
       );