my @arr_in_1 =("User1","user2","user3"); 
my @arr_in_2 =("user2","user3","User4"); 

my @union_arr; 
foreach my $arr_1(@arr_in_1) 
{ 
   push(@union_arr,$arr_1);
} 

foreach my $arr_1(@arr_in_1) 
{ 
  #print "The value in array 1 is :" .lc $arr_1."\n";
  foreach my $arr_2(@arr_in_2) 
  { 
     #print "the value in array 2 is :". lc $arr_2." \n";
     if (lc $arr_2 ne lc $arr_1) 
     { 
        push(@union_arr,$arr_2); 
     } 
  } 
} 

print "union array @union_arr";