use strict;

my @Array1 = ("A", "B", "C", "D");
my @Array2 = ("J", "C", "T", "A");
my %seen = ();
foreach (@Array1) { $seen{$_}++ };
my $count = 0;
foreach (@Array2) { exists $seen{$_} and $count++; }

print "Number of duplicates: $count\n";