$ cat foo.pl
use strict;
use warnings;

my %hash = (
    foo => undef,
    bar => { a=>7 },
    baz => 'quandary',
);

for my $k ('joe', 'foo', 'bar', 'baz') {
    if (! exists $hash{$k}) {
        print "Key $k does not exist in hash\n";
        next;
    }
    if (! defined $hash{$k}) {
        print "Key $k exists, but value is not defined\n";
        next;
    }
    my $r = ref($hash{$k});
    $r = "SCALAR" if $r eq '';
    print "Key $k exists, refers to ", $r, "\n";
    print "looking up \$hash{$k}{a}: $hash{$k}{a}\n";
}

Roboticus@Waubli ~
$ perl foo.pl
Key joe does not exist in hash
Key foo exists, but value is not defined
Key bar exists, refers to HASH
looking up $hash{bar}{a}: 7
Key baz exists, refers to SCALAR
Can't use string ("quandary") as a HASH ref while "strict refs" in use at foo.pl line 22.