"Arrays are for ordering, hashes are for searching."

That's a good thought to keep in mind. Again, presuming what the others have presumed in this thread, that you actually have a hash of arrayrefs leading to the arrays of your data, you are trying to "search" an "array". You can do that badly once, but if you have to do it repeatedly, I suggest you restructure your data as:

my %myhash = (
  key1 => { map {$_ => 1} qw(abd bcd cde def efg) },
  key2 => { map {$_ => 1} qw(abd def} },
  key3 => { map {$_ => 1} qw(efg cde tgh) },
);
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because then you can quickly find all first level keys that have "efg" in the second level with:

my @winners = grep $myhash{$_}{efg}, keys %myhash;
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Hopefully, this gives you an idea about how to restructure it. Now if you actually need the ordering at the second level, there are hybrid solutions. But if all you need is to search that second level, then use hashes there.

And, if you need to turn this "inside out", finding all first-level keys for each second-level item, the syntax is a bit messy but the algorithm is straightforward:

my %inside_out;
for my $first_key (keys %myhash) {
  for my $second_key (keys %{$myhash{$first_key}}) {
    $inside_out{$second_key}{$first_key} = 1;
  }
}
for my $second_key (keys %inside_out) {
  print "$second_key => ",
    join(", ", sort keys %{$inside_out{$second_key}}), "\n";
}
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-- Randal L. Schwartz, Perl hacker
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