lza:

You can't have looked very hard for the answer, as it's a commonly-asked question. Read perldoc perlfaq4.

Update: I was probably expecting a bit much in thinking that beginning programmers would be able to adapt the code in perlfaq4 "How do I compute the difference of two arrays? How do I compute the intersection of two arrays". As pennance, here's a simple adaptation:

#!/usr/bin/perl
use 5.14.0;
use warnings;
use autodie;

my @source=(1, 2, 3, 4, 5, 7);
my @target=(0, 1, 3, 4, 6);
my %count = ();

$count{$_} = 1 for @source;
$count{$_} |= 2 for @target;

my (@not_in_src, @not_in_tgt, @in_both);

for (sort keys %count) {
    push @not_in_tgt, $_ if $count{$_} == 1;
    push @not_in_src, $_ if $count{$_} == 2;
    push @in_both,    $_ if $count{$_} == 3;
}

print "Not in source: ", join(", ", @not_in_src), "\n";
print "Not in target: ", join(", ", @not_in_tgt), "\n";
print "In both: ", join(", ", @in_both), "\n";
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...roboticus

When your only tool is a hammer, all problems look like your thumb.