1) array1:  1,2,5,9,11,12,13
2) array2:  1,2,5,11,12,13
3) array3:  1,2,5,7,9,12,13


##</code><code>##

my %union1;
my %isect1;
                                                                                                          
# FIND THINGS THAT ARE IN BOTH ARRAY1 AND AND ARRAY2

foreach my $e (@array1)  { $union1{$e}=1}
foreach my $g (@array2) {
        if ($union1{$g}) { $isect1{$g}=1}
        $union1{$g}=1;
}
                                                                                                          
my @isect1=keys %isect1;

# FIND THINGS THAT ARE IN ALL THREE ARRAYS
my %union2;
my %isect2;
                                                                                                          
                                                                                                          
foreach my $e (@isect1)  { $union2{$e}=1}
foreach my $g (@array3) {
        if ($union2{$g}) { $isect2{$g}=1}
        $union2{$g}=1;
}
                                                                                                          
# @ISECT2 CONTAINS COMMON VALUES IN ALL THREE ARRAYS
                                                                                                          
my @isect2=keys %isect2;