Another simple way, without deleting all those keys, is simply to check for existence as in the code snippet, below. It's mostly comments. The code is small.

#!/bin/perl

use strict;
use warnings;

# We create two lists
my @list1 = ( 1, 2, 3, 4, 5, 6, 7, 8, 9 );
my @list2 = ( 1, 3, 5, 7, 9, 11, 13, 15 );

# Let's convert them into hashes so
# we can check for existence.  We
# set the value equal to 1, since
# we don't need to count them.
my %hash1 = map { $_ => 1 } @list1;
my %hash2 = map { $_ => 1 } @list2;

# Here's the fun part.  Let's create an array/list
# of all the keys in hash1 that aren't in hash2.
# This is the same as items in list 1 that aren't
# in list 2.
my @list1_minus_list2 = grep { !exists $hash2{$_} } sort keys %hash1;

# Let's do it again.  Let's create an array/list
# of all the keys in hash1 that aren't in hash1.
# This is the same as items in list 2 that aren't
# in list 1.
my @list2_minus_list1 = grep { !exists $hash1{$_} } sort keys %hash2;

# Let's print them out nicely.
print join( ',', @list1_minus_list2 ), "\n";
print join( ',', @list2_minus_list1 ), "\n";

__END__

# Results

2,4,6,8
11,13,15
[download]

Update:To find the intersection, change the !exists to exists. To create a union, simply create one hash from both lists with my %union_hash = map { $_ => 1 } (@list1,@list2);. Then just extract the keys with something like my @union_list = sort keys %union_hash;

Hope this helped,
-v.