#!/bin/perl5

use strict;
use warnings;

my %hash =  ( key1 => 0,
              key2 => 0,
              key3 => 0);

my %dups = reverse %hash;
my $count = keys %dups;
print "$count\n";
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$count = 1

Beaten to the post of course. This is just in case you wanted to do it on two lines instead of one!