use strict;
use warnings;

my @array1 = (1,2,3,4);
my @array2 = (1,5,3,6,8,2,4);
my %hash;

use constant kReplace => 3;

@hash{@array1} = @array1;

my @only2 = grep {! exists $hash{$_}} @array2;

# remove the unwanted "value" from array 2
@array2 = grep {$_ != kReplace} @array2;

# Substitute a new value in array 1
for (@array1) {
    next if $_ != kReplace;
    
    $_ = shift @only2;
    last; # Assuming there will only be one value to replace
}

print "array 1: @array1\n";
print "array 2: @array2\n";