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;

##</code><code>##

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