This is a FAQ. Please see perldoc -q duplicate or perlfaq4, How can I remove duplicate elements from a list or array. Obviously you want to keep just the duplicates instead of removing them, but doing so is left as an exercise.

Thanks! The perlfaq4 example (with the ! for unique removed) is the one that I'll use from now on.

my %seen = ();
my @duplicates = grep { $seen{ $_ }++ } @array;
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That won't work. If there's three of the same thing, it will print the duplicate twice. That's why I used == in my earlier post instead of (an implied) >.

my @a = qw/ 1 2 3 4 5 6 7 8 9 5 7 9 /;
my %h;
$h{$_}++ for @a;
my @dups = grep { $h{$_} > 1 } keys %h;
print "@dups\n";
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Is one way.

Combining your loops into one:

my @a = qw/ 1 2 3 4 5 6 7 8 9 5 7 9 5 /;
my %seen;
my @dups = grep ++$seen{$_}==2, @a;
print "@dups\n";
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That's damned sneaky. I like it++