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

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

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