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";
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# Turn at least one of the arrays into a hash (hashes are perfect for 
+this kind of thing)
@Hash1{"A", "B", "C", "D"} = undef;

# Then, just use exists...
print scalar grep {exists $Hash1{$_}} ("J", "C", "T", "A");
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grep {$x=$_;grep{$_ eq $x} @ar1} @ar2;
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perlfaq4 also has an answer, if the elements of each array are unique. See How do I compute the intersection of two arrays?

my @Array1 = qw( A B C D );
my @Array2 = qw( J C T A );

my $count=0;

for ( @Array1 )
{
    $count++ if $_ ~~ @Array2;
}

print "Count = $count\n";
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It should be noted that '~~', the Smartmatch Operator, is experimental: see "perl5180delta: The smartmatch family of features are now experimental".

I haven't investigated whether it makes a difference with this specific usage; however, it may be worth checking this (from "perlop: Smartmatch Operator"):

"First available in Perl 5.10.1 (the 5.10.0 version behaved differently), ..."

-- Ken

use Benchmark;
@Array1 = ("A", "B", "C", "D");
@Array2 = ("J", "C", "T", "A");
%seen = ();

timethese(1_000_000, {
  'stephen'=> sub {
    %seen=();foreach (@Array1) { $seen{$_}++ };
    my $count = 0;
    foreach (@Array2) { exists $seen{$_} and $count++; }
  },

  'Russ'=> sub {
    @Hash1{@Array1} = undef;
    $matches=scalar grep {exists $Hash1{$_}} @Array2;
  },

  'BUU'=> sub {
    $matches=grep{$x=$_;grep{$_ eq $x} @Array1} @Array2;
  },

  'Perlfaq'=> sub       {
    @union = @intersection = @difference = ();
    %count = ();
    foreach $element (@Array1, @Array2) {$count{$element}++ 
    foreach $element (keys %count) {
      push @union, $element;
      push @{ $count{$element} > 1 ? \@intersection : \@difference }, 
+$element;
    }
  }
});

__DATA__

Benchmark: timing 1000000 iterations of BUU, Perlfaq, Russ, stephen...
       BUU: 14 wallclock secs (15.17 usr +  0.00 sys = 5.17 CPU) @ 659
+19.58/s (n=1000000)
   Perlfaq: 58 wallclock secs (57.13 usr +  0.00 sys = 7.13 CPU) @ 175
+03.94/s (n=1000000)
      Russ:  6 wallclock secs ( 5.12 usr +  0.00 sys = 5.12 CPU) @ 195
+312.50/s (n=1000000)
   stephen: 17 wallclock secs (15.99 usr +  0.01 sys = 6.00 CPU) @ 625
+00.00/s (n=1000000)
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@Array1 = ("A", "B", "C", "D");
@Array2 = ("J", "C", "T", "A");
foreach $e (@Array1, @Array2) {
    $union{$e}++ && $intersect{$e}++
}
@intersect = keys %intersect;
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