Well, I thought of something obvious along the lines of:

my $undefined;
while my $key (qw/A B C/) {
  $undefined = 1, last unless defined $hashref->{$key};
}
if ($undefined) {
  # at least one key undefined
} else {
  # all keys defined
}
[download]

I'd say this is faster than the grep/map approach when the list of keys to test is long. It also depends on the probability of a key being undefined. Try and Benchmark some real-life examples and you get a definitive answer concerning the speed.

-- Hofmator