you can just do my @keys = keys %hash and then apply the Q&A node How can I find the union/difference/intersection of two arrays? to @keys and @all_ids.

To modify your code, though, i would hash up the array first, and then just use the exists function:

my %all_ids = map { $_ => undef } @all_ids;  # note there's multiple w
+ays to init this hash

while ( my ($key, $value) = each(%hash) ) {
  printf "the pair (%s => %s) is not in the array", $key, $value unles
+s exists $all_ids{$key};
}

# or grep instead of while:
my @missing_keys = grep( !exists $all_ids{$_}, keys %hash);
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