$ cat t.pl
use strict;
use warnings;

my $data = { FOO=>{HOST=>undef, BANG=>3}};

for my $K (qw(HOST BANG GONE)) {
    print "$K exists\n"  if exists $data->{FOO}{$K};
    print "$K defined\n" if defined $data->{FOO}{$K};
    print "$K val==''\n" if $data->{FOO}{$K} eq "";
}

$ perl t.pl
HOST exists
Use of uninitialized value in string eq at t.pl line 9.
HOST val==''
BANG exists
BANG defined
Use of uninitialized value in string eq at t.pl line 9.
GONE val==''