#!/usr/bin/env perl
use strict;
use warnings;
use feature 'say';
use Data::Printer;

# populating your hash
my %data = map { chomp; $_ => 1 } <DATA>;
# for your example, might look something like this:
# my %data = map { $_ => 1 } $sth->fetchrow(); # do you mean fetchrow_array?

# two ways to check whether key exists in hash
say "ABC exists" if exists $data{'ABC'};
say "JKL exists" if $data{'JKL'} == 1;

p %data;

__DATA__
ABC
JKL
HGJ
KJI
GHJ
LKJ

##</code><code>##

{
    ABC   1,
    GHJ   1,
    HGJ   1,
    JKL   1,
    KJI   1,
    LKJ   1
}
ABC exists
JKL exists