I don't know of an existing function, and my reduce-fu is weak. But, the 1st thing that comes to mind is a hash with grep:

use warnings;
use strict;

my %uniq;
$uniq{$_}++ for (1 , 2, 3 , 4 , 1, 2, 3);
my $matches = grep { $_ > 1 } values %uniq;
die "Walrus Festival!" if $matches > 2;
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Update: replaced keys with values