#!/usr/bin/perl

use strict;
use warnings;
use Data::Dumper;

my $A = 'KEY';

my $href={
    #HASH => { $A => 1}
};

if (defined($href->{HASH}->{$A}) ){
    print "definiert\n"
}else{
    print "nicht definiert\n"
}

print Dumper($href);