my %seen = (); # use hash for counting elements
my @doubles = grep {
  # if value not in seen, return 0 and add it 
  #    to %seen with start value of 1
  $seen{$_}++ 
} @array1;

foreach my $element ( @doubles ) {
  print "$element\n";
} # foreach @doubles
undef %seen;
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disadvantage: if you have three or more values, they will be printed more than once, so the following solution might be more precise:

my %seen = ();
$seen{$_}++ for @array1;
for my $value ( keys %seen ) {
  print "$value\n" if $seen{$value} > 1;
} # for %seen
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A variation:

use strict;
use warnings;

my @critters = qw(
                  ant camel snake camel llama
                  cow sheep herring camel llama
                  );
my %rec_of;
my %dup_rec_of;

for my $critter (@critters) {
    $dup_rec_of{ $critter }++ if $rec_of{ $critter }++;
}
printf "%s\n", join ', ', sort keys %dup_rec_of;
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which prints:

camel, llama
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Cheers