proving I can still write Perl code after all these years (no googling or stack overflowing allowed). Ugly, but it works!

gistfile1.pl

#!/usr/bin/perl

@list1 = (1,2,3,4,5,6);
@list2 = (3,4,5,10);

intersection(\@list1, \@list2);

sub intersection {
    my ($a,$b) = @_;
    
    my %intersect = ();
    for (@{$a}) { 
        $intersect{$_} = 1;
    }
    for (@{$b}) { 
        if ($intersect{$_} == 1) {
            $intersect{$_} += 1;
        }
    }

    @results = ();
    @keys = keys(%intersect);
    for (@keys) {
        if ($intersect{$_} > 1) { 
            push(@results, $_);
        }
    }

    print "intersection of " . 
        join(',',@{$a}) . ' and ' . join(',',@{$b}) . 
        ' is ' .  join(',', @results) . "\n";
}