Because the value you assign to each key is unimportant, you might as well use undef to signify that:

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
[download]

If you are reading lines from a file you can reduce the amount of memory you use at any one time by assigning the lines to the hash one at a time--rather than storing the lines in an array and then doing a gang assignment like above.