%ids;

#loop through lines
#add id to the list
$ids{$my_input_id} = 'anything - doesn't matter';

#end loop
@ids = keys %ids
join(',' @ids);
[download]

%ids;

#loop through lines
#add id to the list
$ids{$my_input_id} = 'id';

#end loop
@ids = values %ids
join(',' @ids);
[download]

Always use strict and warnings, so you would have to declare your variable with my.

With a hash, the key is unique, but not neccessarily the values.

Peter (Guo) Pei

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.