if (defined $hash->{A} &&
    defined $hash->{B} &&
    defined $hash->{C})
{
}

##</code><code>##

my $hash = { A => 1, B => 1, C => 1, };

if (grep /1/, map { !defined $hash->{$_} } qw(A B C)) {
    print "At least one undefined!\n";
} else {
    print "All there!\n";
}