use warnings;
use strict;
use 5.010;

my @arr = qw{ a a b b c c c };
my %hash;

@hash{@arr} = undef;

use Data::Dumper;
say Dumper(\%hash);


--output:--
$VAR1 = {
          'c' => undef,
          'a' => undef,
          'b' => undef
        };

-----------


if (exists $hash{a} ) {
	say 'yes';
}
else {
	say 'no';
}

--output:--
yes
----------

for (keys %hash) {
	say;
}

--output:--
c
a
b